Convert binding doc ad7879.txt to yaml format.
Additional change:
- Add ref to /schemas/spi/spi-peripheral-props.yaml
- Add #gpio-cell
- Remove  spi-cpol and spi-cpha in example, the place hold 'spi' can't
correct detect spi-controler.yaml. So these two properties can't
be recongnized.

Fix warning:
arch/arm64/boot/dts/freescale/imx8dx-colibri-aster.dtb:
/bus@5a000000/i2c@5a800000/touchscreen@2c: failed to match any schema with compatible: ['adi,ad7879-1']
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240810143840.3615450-1-Frank.Li@nxp.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>