spi: spi-fsl-dspi: Add specific compatibles for all Layerscape SoCs

Make the second compatible string optional for LS1012A, LS1088A and
LS2080A. Old versions of the spi-fsl-dspi.c driver still need to probe
on the old, generic compatible string for these controllers (such as
"fsl,ls1021a-v1.0-dspi") which provides less functionality.

Document the device tree bindings for LS1043A and LS1046A, whose
bindings are already in use in fsl-ls1043a.dtsi and fsl-ls1046a.dtsi.

Introduce new compatible strings for LS1028A and LX2160A. There will be
no second compatible string for these.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Message-Id: <20200302001958.11105-2-olteanv@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>