of: add const to struct *of_device_id.data

Drivers should never need to modify the data of a device id. So it can
be const which in turn allows more consts in the driver.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>