Instead of listing directly properties typical for SPI peripherals,
reference the spi-peripheral-props.yaml schema.  This allows using all
properties typical for SPI-connected devices, even these which device
bindings author did not tried yet.

Remove the spi-* properties which now come via spi-peripheral-props.yaml
schema, except for the cases when device schema adds some constraints
like maximum frequency.

While changing additionalProperties->unevaluatedProperties, put it in
typical place, just before example DTS.

The sitronix,st7735r references also panel-common.yaml and lists
explicitly allowed properties, thus here reference only
spi-peripheral-props.yaml for purpose of documenting the SPI slave
device and bringing spi-max-frequency type validation.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220727164312.385836-1-krzysztof.kozlowski@linaro.org