ARM: OMAP: omap_device: keep track of driver bound status

Use the bus notifier to keep track of driver bound status by adding a
new internal field to struct omap_device: _driver_status.

This will be useful for follow-up patches which need to know whether
or not a driver is bound in order to make intelligent omap_device
enable/idle decisions.

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