ARM: OMAP2+: hwmod: RTC: Add lock and unlock functions

RTC IP have kicker feature which prevents spurious writes to its registers.
In order to write into any of the RTC registers, KICK values has to be
written to KICK registers. Also, RTC busy flag needs to be polled for
non-TC registers as well, without which update is not proper and confirmed
it by testing on DRA7-evm.
Introduce omap_hwmod_rtc_unlock/lock functions, which  writes into these
KICK registers inorder to lock and unlock RTC registers.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[paul@pwsan.com: fixed subject line]
Signed-off-by: Paul Walmsley <paul@pwsan.com>