dt-bindings: pwm: allwinner: Add H6 PWM description

H6 PWM block is basically the same as A20 PWM, except that it also has
bus clock and reset line which needs to be handled accordingly.

Expand Allwinner PWM binding with H6 PWM specifics.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>