The current code use no locking at all, which is obviously not that
great and can lead to concurrency issues, especially with the newer SMP
SoCs from Allwinner.

Add some locking where it's needed.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>