drivers/rtc/rtc-at32ap700x.c: use devm_* APIs

devm_* functions are device managed and make cleanup code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Hans-Christian Egtvedt <hcegtvedt@atmel.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>