Protect any RMW access to the registers by a spinlock.

Fixes: 7e5ea974 ("pinctrl: pinctrl-microchip-sgpio: Add pinctrl driver for Microsemi Serial GPIO")
Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220226204507.2511633-2-michael@walle.ccSigned-off-by: Linus Walleij <linus.walleij@linaro.org>