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

Documentation/devicetree/bindings/spi/spi-xilinx.txt

@@ -8,7 +8,8 @@ Required properties:
 			  number.
 
 Optional properties:
-- xlnx,num-ss-bits	: Number of chip selects used.
+- xlnx,num-ss-bits	 : Number of chip selects used.
+- xlnx,num-transfer-bits : Number of bits per transfer. This will be 8 if not specified
 
 Example:
 	axi_quad_spi@41e00000 {
@@ -17,5 +18,6 @@ Example:
 			interrupts = <0 31 1>;
 			reg = <0x41e00000 0x10000>;
 			xlnx,num-ss-bits = <0x1>;
+			xlnx,num-transfer-bits = <32>;
 	};