rtc: pcf8563: stop caching voltage_low

voltage_low is only updated when reading the time, this means that using
RTC_VL_READ will miss the VL flag if the time has not been read before
using the ioctl. Always read the status from the hardware.

Link: https://lore.kernel.org/r/20191214220259.621996-9-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>