OMAP2+: PM: initial runtime PM core support

OMAP2+: PM: initial runtime PM core support

Implement the new runtime PM framework as a thin layer on top of the
omap_device API.  OMAP specific runtime PM methods are registered with
the as custom methods on the platform_bus.

In order to determine if a device is an omap_device, its parent device
is checked.  All omap_devices have a new 'omap_device_parent_ device
as their parent device, so checking for this parent is used to check
for valid omap_devices.  If a device is an omap_device, then the
appropriate omap_device functions are called for it.  If not, only the
generic runtime PM functions are called.

Device driver's ->runtime_idle() hook is called when the runtime PM
usecount reaches zero for that device.  Driver's ->runtime_suspend()
hooks are called just before the device is disabled (via
omap_device_idle()), and device driver ->runtime_resume() hooks are
called just after device has been enabled (via omap_device_enable().)

OMAP4 build support from Rajendra Nayak <rnayak@ti.com>.
OMAP2 build support from Charulatha V <charu@ti.com>

Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Charulatha V <charu@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>