rtc: add ioctl to get/clear battery low voltage status

Currently there is no generic way to get the RTC battery status within an
application.  So add an ioctl to read the status bit.  The idea is that
the bit is set once a low voltage is detected.  It stays there until it is
reset using the RTC_VL_CLR ioctl.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>