drivers/rtc/rtc-pl030.c: use devm_kzalloc() instead of kmalloc()

In order to be free automatically and make the cleanup paths more simple,
use devm_kzalloc() instead of kmalloc().

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Cc: Joe Perches <joe@perches.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>