rtc: abx80x: Implement RTC_VL_READ,CLR ioctls

Implement standard ioctls for polling the battery status and clearing
the battery low indication from userspace.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>