dt-bindings: pwm: sprd,ums512-pwm: convert to YAML

Convert the Spreadtrum/Unisoc UMS512 PWM controller bindings to DT schema.
Adjust filename to match compatible. Drop assigned-* properties as these
should not be needed.

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Chunyan Zhang <zhang.lyra@gmail.com>
Acked-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/ZyH-JASRcpMXYsmH@standask-GA-A55M-S2HP


[Replaced Baolin Wang's email address in maintainers list]
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>