rtc: make of_device_ids const

of_device_ids (i.e. compatible strings and the respective data) are not
supposed to change at runtime.  All functions working with of_device_ids
provided by <linux/of.h> work with const of_device_ids.  This allows to
mark all struct of_device_id below drivers/rtc const, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>