rtc: sun6i: Add some locking

commit a9422a19 upstream.

Some registers have a read-modify-write access pattern that are not atomic.

Add some locking to prevent from concurrent accesses.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>