SMM: add mainboard_apm_cnt() callback

SMM: add mainboard_apm_cnt() callback

motherboards can use this hook to get notified if someone writes
to the APM_CNT port (0xb2). If the hook returns 1, the chipset
specific hook is also skipped.

Change-Id: I05f1a27cebf9d25db8064f2adfd2a0f5759e48b5
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Reviewed-on: http://review.coreboot.org/3
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Tested-by: build bot (Jenkins)