rtc: pl031: use devm_* for allocating memory and mapping resource

Use the devm_* APIs for allocating memory and mapping the memory in
the probe function to relieve the driver from having to deal with
this in the cleanup paths.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>