ARM: OMAP2+: powerdomain: Introduce cpu_pm notifiers for context save/restore

Inroduce cpu_pm notifiers for context save/restore. This is
needed for am43xx family during rtc only mode with ddr in
self-refresh.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>