dt-bindings: pwm: Update bindings for the Meson G12A Family

Update the doc to explicitly support Meson G12A Family.
The 2 first (A & B) AO PWM uses different clock source than the last 2
(C & D) AO PWM modules, thus we need to differentiate them.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>