davinci: da850/omap-l138: add support for SoC suspend

This patch adds support for registering for suspend-to-RAM
functionality on da850/omap-l138 SoCs.

da850 supports wakeup based on external event and RTC
alarm.

Currently only RTC alarm based wakeup is supported.
Support for wakeup based on external event will be
added as later improvements.

For scheduling an alarm event on RTC some useful code
is present in Documentation/rtc.txt

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>