dt-bindings: ap806: document gpio marvell,pwm-offset property

Update the example as well. Add the '#pwm-cells' and 'clocks' properties
for a complete working example.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>