rtc: rtc-max77686: use devm_rtc_device_register()

devm_rtc_device_register() is device managed and makes cleanup
paths simpler. Also, this patch uses devm_request_threaded_irq().

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