rtc: add new VL flag for backup switchover

A new flag RTC_VL_BACKUP_SWITCH means that a backup switchover happened
since last flag clear.

Link: https://lore.kernel.org/r/20200505201310.255145-1-alexandre.belloni@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

include/uapi/linux/rtc.h

@@ -99,6 +99,7 @@ struct rtc_pll_info {
 #define RTC_VL_BACKUP_LOW	_BITUL(1) /* Backup voltage is low */
 #define RTC_VL_BACKUP_EMPTY	_BITUL(2) /* Backup empty or not present */
 #define RTC_VL_ACCURACY_LOW	_BITUL(3) /* Voltage is low, RTC accuracy is reduced */
+#define RTC_VL_BACKUP_SWITCH	_BITUL(4) /* Backup switchover happened */
 
 #define RTC_VL_READ	_IOR('p', 0x13, unsigned int)	/* Voltage low detection */
 #define RTC_VL_CLR	_IO('p', 0x14)		/* Clear voltage low information */