spi: dt-bindings: arm,pl022: Move child node properties to separate schema

In order to validate SPI peripherals, SPI controller-specific child node
properties need to be in a separate schema, spi-peripheral-props.yaml,
which SPI peripheral schemas reference. Move the arm,pl022 child
properties to their own schema file and add a $ref in
spi-peripheral-props.yaml.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230914190033.1852600-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>