mb/dell: Add Latitude E5420 (Sandy Bridge)

mb/dell: Add Latitude E5420 (Sandy Bridge)

Mainboard is Krug 14". I do not physically have this system; someone
with physical access to one sent me the output of autoport which I then
modified to produce this port. I was also sent the VBT binary, which was
obtained from `/sys/kernel/debug/dri/0/i915_vbt` while running version
A02 of the vendor firmware.

This was originally tested and found to be working as a standalone board
port in Libreboot, but this variant based port in upstream coreboot has
not been tested.

This can be internally flashed by sending a command to the EC, which
causes the EC to pull the FDO pin low and the firmware to skip setting
up any chipset based write protections [1]. The EC is the SMSC MEC5055,
which seems to be compatible with the existing MEC5035 code.

[1] https://gitlab.com/nic3-14159/dell-flash-unlock

Change-Id: I0283653156083768e1fd451bcf539b4e028589f4
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82129
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>