soc/amd/mendocino: Add support for separate RW A/B partition SPL file

soc/amd/mendocino: Add support for separate RW A/B partition SPL file

Add support for having different Security Patch Level (SPL) table files
in the read-only and the read-write A/B partitions. This allows the SPL
table file in the main or RO FMAP partition to only cover the embedded
firmware binaries in that partition and have a separate SPL file in the
RW A and B partitions that covers the embedded firmware binaries in the
RW partitions.

BUG=b:243470283

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I1ba8c370ce14f7ec88e7ef2f9d0b64d6bb4fa176
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67555
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martin.roth@amd.corp-partner.google.com>