dt-bindings: pxa-pwm: Convert to YAML

Convert the PXA PWM binding file from TXT to YAML.

The original binding does not mention any clocks, but the PWM controller
will not probe without a clock.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Duje Mihanović <duje.mihanovic@skole.hr>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240107-pxa-pwm-yaml-v3-1-92ac90911c3f@skole.hr


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>