dt-bindings: spi: spi-mux: rename flash node

The recent conversion of the common MTD properties to YAML now mandates
a particular node name for SPI flash devices.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210517153946.9502-1-michael@walle.cc


Signed-off-by: Mark Brown <broonie@kernel.org>