spi: pl022: Document property values

The property "pl022,com-mode" can only assume one of the values of
the enum ssp_mode, defined in include/linux/amba/pl022.h
List the possible numeric values and report the associated meaning.
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi_pl022.txt

@@ -30,7 +30,10 @@ contain the following properties.
 	0: SPI
 	1: Texas Instruments Synchronous Serial Frame Format
 	2: Microwire (Half Duplex)
-- pl022,com-mode : polling, interrupt or dma
+- pl022,com-mode : specifies the transfer mode:
+	0: interrupt mode
+	1: polling mode (default mode if property not present)
+	2: DMA mode
 - pl022,rx-level-trig : Rx FIFO watermark level
 - pl022,tx-level-trig : Tx FIFO watermark level
 - pl022,ctrl-len : Microwire interface: Control length