spi: xilinx: Add DT support for selecting transfer word width

This core supports either 8, 16 or 32 bits as word width. This value is only
settable on instantiation, and thus we need to support any of them by means
of the device tree.

Signed-off-by: Alvaro Gamez Machado <alvaro.gamez@hazent.com>
Link: https://lore.kernel.org/r/20191024110757.25820-3-alvaro.gamez@hazent.com


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