spi: xilinx: add description of new property xlnx,num-transfer-bits

This property is used to set the number of bits per transfer (bits_per_word).

Xilinx' IP core allows either 8, 16 or 32, and is non changeable on runtime,
only when instantiating the core.

Signed-off-by: Alvaro Gamez Machado <alvaro.gamez@hazent.com>
Link: https://lore.kernel.org/r/20191024110757.25820-2-alvaro.gamez@hazent.com


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