ARM: OMAP: Activate MPU retention code in omap2/pm.c

FIXME section in function omap2_allow_mpu_retention didn't allow MPU to
enter into retention even if system was idle. MPU retention works fine at
least in Nokia N800.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>