southbridge/intel/lynxpoint: refactor rtc failure checking

In order to prepare for checking RTC failure in the early boot
paths move the rtc failure calculation to pmutil.c and add a helper
function to determine if failure occurred.

BUG=b:63054105

Change-Id: I368c31b9935c0fa9e8a1be416435dd76f44ec1ec
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/21557
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/southbridge/intel/lynxpoint/pmutil.c

@@ -553,3 +553,13 @@ void disable_gpe(u32 mask)
 	gpe0_en &= ~mask;
 	outl(gpe0_en, get_pmbase() + gpe0_reg);
 }
+
+int rtc_failure(void)
+{
+#if defined(__SIMPLE_DEVICE__)
+	device_t dev = PCI_DEV(0, 31, 0);
+#else
+	device_t dev = dev_find_slot(0, PCI_DEVFN(31, 0));
+#endif
+	return !!(pci_read_config8(dev, GEN_PMCON_3) & RTC_BATTERY_DEAD);
+}