dt-bindings: pwm: sunxi: Add new compatible strings

The PWM controllers found in the Allwinner A64 and H5 SoCs are fully
compatible to the PWM controllers found in the A13 and H3.
Add new compatible strings for those SoCs to the binding document, so
that they can be safely used, together with a fallback string
(preferably "allwinner,sun5i-a13-pwm").

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>