src/soc/stoneyridge: Add a check for CMOS failure

BUG=b:77345148
TEST=Pull power from grunt, verify CMOS power failure is detected.
Reboot and verify that CMOS power failure is not detected.

Change-Id: Idbf0254e197a6d282e618a98bced52ea5a44917f
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/25468
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Marc Jones <marc@marcjonesconsulting.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/stoneyridge/pmutil.c

@@ -17,11 +17,12 @@
 #include <soc/southbridge.h>
 #include <security/vboot/vboot_common.h>
 #include <security/vboot/vbnv.h>
+#include <pc80/mc146818rtc.h>
 
 int vbnv_cmos_failed(void)
 {
-	/* FIXME: RTC failure checking not supported. */
-	return 0;
+	/* If CMOS power has failed, the century will be set to 0xff */
+	return cmos_read(RTC_CLK_ALTCENTURY) == 0xff;
 }
 
 int vboot_platform_is_resuming(void)