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