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

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) <robh@kernel.org>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://patch.msgid.link/20250715202711.1882103-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>