rtc: add devm_rtc_device_{register,unregister}()

These functions allow the driver core to automatically clean up any
allocation made by rtc drivers.  Thus it simplifies the error paths.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>