dt-bindings: timer: Add PWM compatible for X1000 SoC

The PWM hardware on the X1000 SoC is almost identical to other
Ingenic SoCs, so it can be used with only minor driver changes.

Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>