soc/intel: Add get_pmbase

soc/intel: Add get_pmbase

Originally a part of security/intel/stm.

Add get_pmbase to the intel platform setup code.

get_pmbase is used by the coreboot STM setup functions to ensure
that the pmbase is accessable by the SMI handler during runtime.
The pmbase has to be accounted for in the BIOS resource list so
that the SMI handler is allowed this access.

Change-Id: If6f6295c5eba9eb20e57ab56e7f965c8879e93d2
Signed-off-by: Eugene D. Myers <edmyers@tycho.nsa.gov>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37990
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>