OMAP3: PM: fix save secure RAM to restore MPU power state

Currently, on HS/EMU devices, MPU power state forced to on during PM
init by the save secure RAM code.  Rather than forcing the state of
MPU powerdomain to on, simply read the current value and restore it
after the ROM code has run.

This only affects the !CPUidle case since when CPUidle is enabled, the
MPU power state is dynamically changed by CPUidle.  In the !CPUidle
case, MPU power state is initialized once at init and never touched.

Acked-by: Tero Kristo <tero.kristo@nokia.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>