Convert binding doc spi-sc18is602.txt (I2C to SPI bridge) to yaml.

Additional change:
- ref spi-controller.yaml

Fix below warning:
arch/arm64/boot/dts/freescale/fsl-lx2160a-bluebox3.dtb:
/soc/i2c@2000000/i2c-mux@77/i2c@7/i2c-mux@75/i2c@0/spi@28: failed to match any schema with compatible: ['nxp,sc18is602b']
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20240813154444.3886690-1-Frank.Li@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>