spi: Move cadence-quadspi.txt to Documentation/devicetree/bindings/spi

Move the Documentation/devicetree/bindings/mtd/cadence-quadspi.txt to
Documentation/devicetree/bindings/spi/
Signed-off-by: Ramuthevar Vadivel Murugan <vadivel.muruganx.ramuthevar@linux.intel.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201124041840.31066-5-vadivel.muruganx.ramuthevar@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>