vendorcode/amd/agesa: Make Porting.h common between families

Change-Id: Ica17b2452498f30b710533caf610c9f0c1a0452c
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7594
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)