spi: dt-bindings: document zero value for spi-{rx,tx}-bus-width properties

spi: dt-bindings: document zero value for spi-{rx,tx}-bus-width properties

Following a change to the SPI framework, providing a value of zero for
'spi-rx-bus-width' and 'spi-tx-bus-width' is now possible and will
essentially mean that no RX or TX is allowed.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20201221152936.53873-3-alexandru.ardelean@analog.com
Signed-off-by: Mark Brown <broonie@kernel.org>