rtc: pl031: switch to devm_rtc_allocate_device/rtc_register_device

Switch to devm_rtc_allocate_device to simplify the erro and driver removal
paths.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>