OMAP: omap_device: decouple platform_device from omap_device

Rather than embedding a struct platform_device inside a struct
omap_device, decouple them, leaving only a pointer to the
platform_device inside the omap_device.

Use the arch-specific data field of the platform_device (pdev_archdata)
to add an omap_device pointer after the platform_device has been created.

Signed-off-by: Kevin Hilman <khilman@ti.com>