pinctrl: meson: meson-gxl: add the pwm_ao_b pin

pinctrl: meson: meson-gxl: add the pwm_ao_b pin

This adds support for the pwm_ao_b pin. Unfortunately the registers for
the pwm_ao pins are not documented at all. The source for the pwm_ao_b
pin from this patch is the Khadas VIM GPL kernel source, which sets bit
3 and unsets bits 4 and 31 to enable the PWM LEDs.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>