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>