ARM64: dts: meson-gx: add the missing pwm_AO_ab node

All Meson GX SoCs (GXBB, GXL and GXM) have a PWM controller within the
AO domain. When one of the board's LEDs is connected to one of the AO
PWM pins then this can be used to dim that LED (when the leds-pwm driver
is used).
Add the pwm_AO_ab to allow such devices to use the leds-pwm driver.

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