ARM: OMAP: omap_device: Add a method to build an omap_device from a DT node

Add a notifier called during device_add phase. If an of_node is present,
retrieve the hwmod entry in order to populate properly the omap_device
structure.

For the moment the resource from the device-tree are overloaded.
DT does not support named resource yet, and thus, most driver will not
work without that information.

Add a documentation to capture the specifics OMAP bindings needed for
device-tree support.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kevin Hilman <khilman@ti.com>