dt-bindings: pwm: imx: remove interrupt property from required

The driver "drivers/pwm/pwm-imx27.c" never use interrupt. Generally pwm
hardware generate a waveform according to register timing setting. Needn't
interrupt handle at all. So remove it from "required" list.

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240605220839.1398872-1-Frank.Li@nxp.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>