Create simple omap_devices for the main processors and busses.

This is required to support the forth-coming device-based OPP
approach, where OPPs are managed and tracked at the device level.

Also, move these common PM init functions into a common_pm_init call
that is called as a device_initcall().  The PM init is done at this level
to ensure that the driver core is initialized before initialized.
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
[paul@pwsan.com: sparse warnings cleaned up; newly-created functions moved
 from mach-omap2/io.c to mach-omap2/pm.c; newly-created functions renamed
 to start with "omap2" rather than "omap"]
Signed-off-by: Paul Walmsley <paul@pwsan.com>