vc/amd/fsp: Make common directory

vc/amd/fsp: Make common directory

The common directory is for files that shouldn't change, or shouldn't
change much between platforms.

These will be removed from other directories and used in upcoming
commits.

Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Change-Id: I37ed98a67b066598fdebcc5b034e64dc639fda7f
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68191
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Fred Reitberger <reitbergerfred@gmail.com>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>