rtc/mc13783: protect rtc {,un}registration by mc13783 lock

This is to protect from interrupt handlers using an unregistered rtc
device.

To assert that the reset irq is considered now before the rtc is
registered the corresponding status is checked before.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Paul Gortmaker <p_gortmaker@yahoo.com>
Cc: Valentin Longchamp <valentin.longchamp@epfl.ch>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Luotao Fu <l.fu@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>