rtc: jz4740: rework invalid time detection

The scratchpad register is used to detect an invalid time when power to the
RTC has been lost. Instead of deleting that precious information and set
the time to the UNIX epoch, forward it to userspace.

Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>