OMAP2+ / PM: move runtime PM implementation to use device power domains

In commit 7538e3db (PM: add support
for device power domains) a better way for handling platform-specific
power hooks was introduced.

Rather than using the platform_bus dev_pm_ops overrides
(platform_bus_set_pm_ops()), this patch moves the OMAP runtime PM
implementation over to using device power domains.

Since OMAP is the only user of platform_bus_set_pm_ops(), that
interface can be removed (and will be in a forthcoming patch.)

[rjw: Rebased on top of a previous change modifying the handling of
 power domains by the PM core so that power domain callbacks take
 precendence over subsystem-level PM callbacks.]

Signed-off-by: Kevin Hilman <khilman@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>