Documentation: Remove confusing xyz0 naming convention for Lenovo devices

Replace xx30 with Ivy_Bridge and xx20 with Sandy_Bridge.
Also add a note that the Ivy_Bridge tutorial doesn't covert T430s and T431s.

Change-Id: I0b65bca83195ec22cc139130e7cb6183c0972484
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/38929
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Peter Lemenkov <lemenkov@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/lenovo/Sandy_Bridge_series.md

@@ -0,0 +1,48 @@
+# Lenovo Sandy Bridge series
+
+## Flashing coreboot
+```eval_rst
++---------------------+--------------------+
+| Type                | Value              |
++=====================+====================+
+| Socketed flash      | no                 |
++---------------------+--------------------+
+| Size                | 8 MiB              |
++---------------------+--------------------+
+| In circuit flashing | Yes                |
++---------------------+--------------------+
+| Package             | SOIC-8             |
++---------------------+--------------------+
+| Write protection    | No                 |
++---------------------+--------------------+
+| Dual BIOS feature   | No                 |
++---------------------+--------------------+
+| Internal flashing   | Yes                |
++---------------------+--------------------+
+```
+
+## Installation instructions
+* Update the EC firmware, as there's no support for EC updates in coreboot.
+* Do **NOT** accidently swap pins or power on the board while a SPI flasher
+  is connected. It will destroy your device.
+* It's recommended to only flash the BIOS region. In that case you don't
+  need to extract blobs from vendor firmware.
+  If you want to flash the whole chip, you need blobs when building
+  coreboot.
+* The shipped *Flash layout* allocates 3MiB to the BIOS region, which is the space
+  usable by coreboot.
+* ROM chip size should be set to 8MiB.
+
+```eval_rst
+Please also have a look at :doc:`../../flash_tutorial/index`.
+```
+
+## Flash layout
+There's one 8MiB flash which contains IFD, GBE, ME and BIOS regions.
+On Lenovo's UEFI the EC firmware update is placed at the start of the BIOS
+region. The update is then written into the EC once.
+
+![][fl]
+
+[fl]: flashlayout_Sandy_Bridge.svg
+