ec/google/wilco: Add function to indicate if EC uses signed FW

ec/google/wilco: Add function to indicate if EC uses signed FW

This will be used to distinguish the mainboard SKU so that the
correct EC firmware can be bundled with the board.

This is read from EC RAM so it can be used by an ACPI method in
the future.

BUG=b:119490232

Change-Id: I71b8017fc4b88e793dfe709e1cb1ab0f0bcdc4fa
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32001
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>