amd/stoneyridge: Add uart.c to smm-y list

This corrects a build error when a developer needs DEBUG_SMM and one
of the APU's internal UARTs is used.

Change-Id: Ie1962e969a8cb93eefc0b86bf4062752580e5acd
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/21740
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Marc Jones <marc@marcjonesconsulting.com>

src/soc/amd/stoneyridge/Makefile.inc

@@ -93,6 +93,7 @@ ramstage-y += tsc_freq.c
 smm-y += smihandler.c
 smm-y += smi_util.c
 smm-y += tsc_freq.c
+smm-$(CONFIG_DEBUG_SMI) += uart.c
 
 CPPFLAGS_common += -I$(src)/soc/amd/stoneyridge
 CPPFLAGS_common += -I$(src)/soc/amd/stoneyridge/include