spi: dt-bindings: spi-mux: Drop "spi-max-frequency" as required

There's little reason to require the SPI mux to define a maximum bus
frequency as the muxing is just the chip select and devices still define
their maximum freq. In fact, several users don't set "spi-max-frequency"
which caused warnings.

Signed-off-by: Rob Herring (Arm) <[e-mail]>
Reviewed-by: Chris Packham <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/spi/spi-mux.yaml

···
 required:
   - compatible
   - reg
-  - spi-max-frequency
   - mux-controls
 
 unevaluatedProperties: false