dt-bindings: pwm: Convert lpc32xx-pwm.txt to yaml format

Convert pc32xx-pwm.txt to yaml format.

Additional changes:
- add ref to pwm.yaml
- add clocks
- restrict #pwm-cells to 3

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250625161909.2541315-1-Frank.Li@nxp.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>