drivers/rtc/rtc-tps65910.c: enable RTC power domain on initialization

drivers/rtc/rtc-tps65910.c: enable RTC power domain on initialization

Enabling RTC HW block depends on the default value of TPS65910 register.
In some mode, RTC block is disabled by default.(eg.  AM3517 Craneboard) In
this case, RTC_PWDN(RTC power down) bit should be cleared to enable the
RTC HW block.

This patch also works in case that RTC block is active by default, because
there is no side effect even if the bit is updated again.

Tested on AM3517 Craneboard.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Sivaram Nair <sivaramn@nvidia.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>