spi: spi-fsl-dspi: LS2080A and LX2160A support XSPI mode

XSPI allows for 2 extra features:
- Command cycling (use a single TX command with more than 1 word in the
  TX FIFO).
- Increased word size (from 16 bits to 32 bits)

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-Id: <20200302001958.11105-5-olteanv@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>