mb/lenovo/x220: Add x1 as a variant

ThinkPad X1 ( https://www.thinkwiki.org/wiki/Category:X1 ) is nearly a
clone of X220, with additional USB3 controller on pci-e (as i7 variant
of x220), and a powered ESATA port wired to ata4 (Linux' annotation).

Documentation added.

Tested:
- CPU i5-2520M
- Slotted DIMM 8GiB
- Camera
- Mini pci-e on wlan slot
- Msata on wwan slot
- On board SDHCI connected to pci-e
- USB3 controller connected to pci-e
- NVRAM options for North and South bridges
- S3
- TPM1 on LPC
- Linux 4.9.110-3 within Debian GNU/Linux stable, loaded from
  SeaBIOS, or Linux payload (Heads)

Not tested:
- Fingerprint reader on USB2
- Onboard USB2 interfaces (wlan slot, wwan slot)

Change-Id: Ibbc45f22c63b77ac95c188db825d0d7e2b03d2d1
Signed-off-by: Bill XIE <persmule@gmail.com>
Reviewed-on: https://review.coreboot.org/c/29434
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/lenovo/x1.md

@@ -0,0 +1,24 @@
+# Lenovo X1
+
+## Flashing instructions
+
+![x1_flash_ic](x1_flash_ic.jpg)
+
+You have to remove the keyboard in order to access the flash IC (the chip
+inside the red circle on the picture above), as it is under the wider
+cable (already detached from MB in the picture) connecting the keyboard
+to the mainboard.
+
+The flash IC can be a SOIC-8 one or a WSON-8 one, and may be covered with
+a piece of insulation tape.
+
+For more details have a look at [T420 / T520 / X220 / T420s / W520 common] and
+
+```eval_rst
+:doc:`../../flash_tutorial/ext_power`
+```
+
+Steps to access the flash IC are described here [X2xx series].
+
+[X2xx series]: x2xx_series.md
+[T420 / T520 / X220 / T420s / W520 common]: xx20_series.md