dt-bindings: pwm: allwinner: add v3s pwm compatible

The Allwinner V3s pwm peripheral is compatible with the pwm peripheral
inside the Allwinner A20.
This patch adds a compatible string for it.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20210513203527.2072090-2-t.schramm@manjaro.org