mb/google/skywalker: Enable RTC boot init

Enable RTC to get correct timestamp.

BUG=b:379008996
BRANCH=none
TEST=build passed and check RTC time is increasing by command 'cat
/proc/driver/rtc'

skywalker-rev1 ~ # cat /proc/driver/rtc
rtc_time        : 12:36:25
skywalker-rev1 ~ # cat /proc/driver/rtc
rtc_time        : 12:36:28

Change-Id: Idfe6185b9e2ad8d116da454c8d95ddeb32a5998d
Signed-off-by: Shunxi Zhang <ot_shunxi.zhang@mediatek.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/87707
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Yidi Lin <yidilin@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/skywalker/Kconfig

@@ -28,6 +28,7 @@ config BOARD_SPECIFIC_OPTIONS
 	select EC_GOOGLE_CHROMEEC_SPI
 	select FW_CONFIG
 	select FW_CONFIG_SOURCE_CHROMEEC_CBI
+	select RTC
 
 config MAINBOARD_DIR
 	string

src/mainboard/google/skywalker/romstage.c

@@ -5,11 +5,13 @@
 #include <soc/mt6315.h>
 #include <soc/mt6359p.h>
 #include <soc/pmif.h>
+#include <soc/rtc.h>
 
 void platform_romstage_main(void)
 {
 	mtk_pmif_init();
 	mt6315_init();
 	mt6359p_init();
+	rtc_boot();
 	mtk_dram_init();
 }