gpio: pca953x: disable regmap locking

This driver uses its own locking but regmap silently uses a mutex for
all operations too. Add the option to disable locking to the regmap
config struct.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Linus Walleij <linus.walleij@linaro.org>