pinctrl-bcm2835.c: fix race condition when setting gpio dir

In the past setting the pin direction called pinctrl_gpio_direction()
which uses a mutex to serialize this. That was changed to set the
direction directly in the pin controller driver, but that lost the
serialization mechanism. Since the direction of multiple pins are in
the same register you can have a race condition, something that was
in fact observed with the cec-gpio driver.

Add a new spinlock to serialize writing to the FSEL registers.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 1a4541b6

 ("pinctrl-bcm2835: don't call pinctrl_gpio_direction()")
Link: https://lore.kernel.org/r/4302b66b-ca20-0f19-d2aa-ee8661118863@xs4all.nl
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>