sb/intel/common: Make linking rtc.c conditional

Change-Id: I7321da453c0d9bb4a142c3c93103d8dc0ff416b7
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33201
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/soc/intel/fsp_broadwell_de/Kconfig

@@ -15,6 +15,7 @@ config CPU_SPECIFIC_OPTIONS
 	select SOUTHBRIDGE_INTEL_COMMON
 	select SOUTHBRIDGE_INTEL_COMMON_SPI
 	select SOUTHBRIDGE_INTEL_COMMON_RESET
+	select SOUTHBRIDGE_INTEL_COMMON_RTC
 	select PARALLEL_MP
 	select SMP
 	select IOAPIC