dt-bindings: pwm: meson: Add compatible for gxbb ao PWMs

Add compatible string to properly handle the PWMs found in the AO domain
of the gxbb (and gxl) family.

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/pwm-meson.txt

@@ -2,7 +2,9 @@ Amlogic Meson PWM Controller
 ============================
 
 Required properties:
-- compatible: Shall contain "amlogic,meson8b-pwm" or "amlogic,meson-gxbb-pwm".
+- compatible: Shall contain "amlogic,meson8b-pwm"
+                         or "amlogic,meson-gxbb-pwm"
+                         or "amlogic,meson-gxbb-ao-pwm"
 - #pwm-cells: Should be 3. See pwm.txt in this directory for a description of
   the cells format.