rtc: tps65910: add RTC driver for TPS65910 PMIC RTC

TPS65910 PMIC is a MFD with RTC as one of the device.  Adding RTC driver
for supporting RTC device present inside TPS65910 PMIC.

Only support for RTC alarm is implemented as part of this patch.

Signed-off-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>