mb/google/skyrim: Add new mainboard

mb/google/skyrim: Add new mainboard

Skyrim is a new Google mainboard with AMD Sabrina SOC.

BUG=b:214413553
TEST=util/abuild/abuild -t GOOGLE_SKYRIM --clean

Change-Id: I008fea4aa163b8aa66e86735b29b3fdc4e08a327
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61565
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Jon Murphy <jpmurphy@google.com>