ARM64: dts: meson-gxl: add the pwm_ao_b pin

This adds the pwm_ao_b pin to allow boards which have an LED connected
to GPIOAO_9 to use the leds-pwm driver (by activating the pwm_AO_ab node
and passing the pwm_ao_b_pin pinctrl-reference).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>