doc/mb/upsquared: Add documentation

This patch adds documentation about the UP² mainboard
and the IFWI used by Apollolake platform.

Change-Id: Ic708ddbd2616eee4e5ec2740b3eac18b408bde38
Signed-off-by: Felix Singer <felix.singer@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32935
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/soc/intel/apollolake/index.md

@@ -0,0 +1,17 @@
+# Apollolake
+## SPI flash layout
+
+![][apl_flash_layout]
+
+With Apollolake Intel invented another flash layout for x86 firmware called IFWI (Intel FirmWare Image).
+
+Usually on x86 platforms the bootblock is stored at the end of the bios region
+and the Intel ME / TXE has its own IFD region. On Apollolake both have been
+moved into the IFWI region, which is a subregion of "BIOS", since it allows to
+store multiple firmware components.
+
+The IFWI region can be manipulated by `ifwitool`.
+
+[apl_flash_layout]: flash_layout.svg
+
+