OMAP2/3: GPIO: generalize prepare for idle

Currently, the GPIO 'prepare' hook is only called when going to
off-mode, while the function is called 'prepare_for_retention.'  This
patch renames the function to 'prepare_for_idle' and calls it for any
powersate != PWRDM_POWER_ON passing in the powerstate.

The hook itself is then responsible for doing various preparation
based on the powerstate.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>