asus/m5a88-v: Fix build

We added some new flag for certain AMD boards after support for
this board was submitted. Also integrate the mptable refactorings
that happened in the meantime.

Change-Id: I50cf50f343a740832fd1a14a2a1ef5b903315675
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/353
Tested-by: build bot (Jenkins)
Reviewed-by: Sven Schnelle <svens@stackframe.org>