gpio: gpio-ml-ioh: Use spinlock for register access protection

gpio_chip.can_sleep is 0, but current code uses mutex in ioh_gpio_set,
ioh_gpio_get and ioh_gpio_direction_input functions.
Thus those functions are not callable from interrupt context.
This patch converts mutex into spinlock.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>