Convert binding doc rotary-encoder.txt to yaml format.

Additional change:
- Only keep one example.

Fix below warning:
arch/arm64/boot/dts/freescale/imx8mn-dimonoff-gateway-evk.dtb: /rotary-encoder:
    failed to match any schema with compatible: ['rotary-encoder']
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20240811214656.3773098-1-Frank.Li@nxp.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>