OMAP: omap_device: add to_omap_device() macro

Following the model of to_platform_device(), add to_omap_device()
macro so a platform_device pointer can be converted into an
omap_device pointer.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>