soc/amd/cezanne: Include gpio.c in smm

Mainboards can configure gpios in their smihandler.

BUG=b:180507707
TEST=builds

Signed-off-by: Mathew King <mathewk@chromium.org>
Change-Id: I6c2b28f981f580cfb6f982a2d7e4c309d6f82e0b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51263
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>

src/soc/amd/cezanne/Makefile.inc

@@ -36,6 +36,7 @@ ramstage-y	+= reset.c
 ramstage-y	+= root_complex.c
 ramstage-y	+= uart.c
 
+smm-y		+= gpio.c
 smm-y		+= smihandler.c
 smm-y		+= smu.c
 smm-$(CONFIG_DEBUG_SMI) += uart.c