spi: spi-mux: Add reference to spi-peripheral-props.yaml schema

The spi-mux serves as both a SPI peripheral and controller, so add a
reference to spi-peripheral-props.yaml in addition to
spi-controller.yaml. This is necessary to avoid unevaluatedProperties
warnings once those are fully enabled.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20220105183234.3426649-1-robh@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>