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)

src/include/cpu/x86/smm.h

@@ -263,4 +263,4 @@ void __attribute__((weak)) northbridge_smi_handler(unsigned int node, smm_state_
 void __attribute__((weak)) southbridge_smi_handler(unsigned int node, smm_state_save_area_t *state_save);
 
 void __attribute__((weak)) mainboard_smi_gpi(u16 gpi_sts);
-
+int __attribute__((weak)) mainboard_apm_cnt(u8 data);

src/southbridge/intel/i82801gx/smihandler.c

@@ -362,6 +362,9 @@ static void southbridge_smi_apmc(unsigned int node, smm_state_save_area_t *state
 	/* Emulate B2 register as the FADT / Linux expects it */
 
 	reg8 = inb(APM_CNT);
+	if (mainboard_apm_cnt && mainboard_apm_cnt(reg8))
+		return;
+
 	switch (reg8) {
 	case CST_CONTROL:
 		/* Calling this function seems to cause