ARM: OMAP2+: PM/powerdomain: move omap_set_pwrdm_state() to powerdomain code

Move omap_set_pwrdm_state() from the PM code to the powerdomain code,
and refactor it to split it up into several functions.  A subsequent patch
will rename it to conform with the existing powerdomain function names.

This version includes some additional documentation, based on a
suggestion from Jean Pihet.  It also modifies omap_set_pwrdm_state()
to not bail out early unless both the powerdomain current power state
and the next power state are equal.  (Previously it would terminate
early if the next power state was equal to the target power state,
which was insufficiently rigorous.)

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Jean Pihet <jean.pihet@newoldbits.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Tero Kristo <t-kristo@ti.com>