ARM: OMAP2xxx: PM: clean up some crufty powerstate programming code

Don't attempt to put clockdomains to sleep; this should be handled by the
clock framework.  It should be enough to program the next-power-state,
and then let the code in omap_pm_clkdms_setup() deal with the rest.

Start out by programming the MPU and CORE powerdomains to stay ON.
Then control the MPU and CORE powerdomain states directly in
omap2_enter_full_retention() and omap2_enter_mpu_retention().  Not the
most optimal way to do it, but certainly is the most conservative until
OMAP2xxx PM is working again.

Get rid of the open-coded PM_PWSTCTRL_MPU writes in
omap2_enter_mpu_retention(); use the powerdomain code instead.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>