soc/amd/stoneyridge: Add GPIO functions to SMM

GPIO functions are required by the Grunt SMI handler.

Change-Id: Id729139b02c10bdd922b3df298f8f9feb1aff745
Signed-off-by: Marc Jones <marcj303@gmail.com>
Reviewed-on: https://review.coreboot.org/27485
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/soc/amd/stoneyridge/Makefile.inc

@@ -126,6 +126,7 @@ smm-y += tsc_freq.c
 smm-$(CONFIG_DEBUG_SMI) += uart.c
 smm-$(CONFIG_SPI_FLASH) += spi.c
 smm-y += nb_util.c
+smm-y += gpio.c
 
 CPPFLAGS_common += -I$(src)/soc/amd/stoneyridge
 CPPFLAGS_common += -I$(src)/soc/amd/stoneyridge/include