mainboard/lenovo/t430s: Add ThinkPad T431s as a variant

The code is based on autoport and that for T430s

Tested:
- CPU i5-3337U
- Slotted DIMM 2GiB
- Soldered RAM 4GiB from samsung (There may be more models here)
- Camera
- pci-e and usb2 on M.2 slot with A key for wlan
- sata and usb2  (no superspeed components) on M.2 slot with B key for wwan
- On board SDHCI connected to pci-e
- USB3 ports
- libgfxinit-based graphic init
- NVRAM options for North and South bridges
- Sound
- Thinkpad EC
- S3
- TPM1 on LPC
- EHCI debug on SSP2 (USB3 port on the left)
- Linux 4.9.110-3 within Debian GNU/Linux stable, loaded from
  Linux payload (Heads), Seabios may also work.

Not tested:
- Fingerprint reader on USB2 (not present on mine)
- Keyboard backlight (not present on mine)
- "sticky_fn" flag in nvram

Not implemented yet:
- Fn locking in nvram (may not be identical to "sticky_fn")
- C-based native graphic init (since T431s has eDP instead of LVDS)
- Detecting the model of Soldered RAM at runtime, and loading the
  corresponding SPD datum (3 observed) from CBFS (the mechanism may be
  similar to that on x1_carbon_gen1 and s230u, but I do not know how
  to find gpio ports for that, and SPD data stored in vendor firmware.)

Change-Id: Ic8062cacf5e8232405bb5757e1b1d063541f354a
Signed-off-by: Bill XIE <persmule@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30021
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/index.md

@@ -60,6 +60,7 @@ The boards in this section are not real mainboards, but emulators.
 - [T530](lenovo/w530.md)
 - [W530](lenovo/w530.md)
 - [T430 / T530 / X230 / W530 common](lenovo/xx30_series.md)
+- [T431s](lenovo/t431s.md)
 
 ## SiFive
 

Documentation/mainboard/lenovo/t431s.md

@@ -0,0 +1,42 @@
+# Lenovo T431s
+
+## Disassembly Instructions
+
+You must remove the following parts before flipping the mainboard
+off the main frame:
+
+![t431s_bc_removed](t431s_bc_removed.jpg)
+
+* Base cover
+* Hard disk drive
+* Battery pack
+* Keyboard
+
+Its [Hardware Maintenance Manual](https://thinkpads.com/support/hmm/hmm_pdf/t431s_hmm_en_0c10894_02.pdf) could be used as a guidance of disassembly.
+
+![t431s_flash_chip](t431s_flash_chip.jpg)
+
+The WSON-8 flash chip (surrounded with red circle in the photo above)
+sits on the opposite side of the mainboard, under a piece of insulating
+tape. If solders between the chip and soldering pads fortunately
+overflows beside the chip as tiny tin balls attached to soldering pads,
+it will be possible to use a pomona 5250 clip to hold the chip, with
+its metal tips just attached to tin balls, thus connecting the chip to
+the programmer.
+
+![t431s_programming](t431s_programming.jpg)
+
+```eval_rst
+:doc:`../../flash_tutorial/ext_power`
+```
+
+Currently, detecting the model of soldered RAM at runtime and loading
+the corresponding SPD datum from CBFS is not implemented yet. You may
+have to dump the SPD data when running the vendor firmware with
+inteltool, and replace the content of the SPD hex with what is dumped.
+
+(the mechanism may be similar to that on x1_carbon_gen1 and s230u, but
+I do not know how to find gpio ports for that, and SPD data stored in
+vendor firmware.)
+
+[T420 / T520 / X220 / T420s / W520 common]: xx20_series.md