mb/google/rex: Initial setup for ramstage/early gpio config

This adds the initial gpio configuration for the rex initial variant.

BUG=b:238165977
TEST=Boots and no errors on simics

Change-Id: I55ab31c7943e22df9cec8db4a9f0c3ab6f065ae1
Signed-off-by: Tarun Tuli <taruntuli@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65952
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ivy Jian <ivy.jian@quanta.corp-partner.google.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>