spi: pxa2xx: dt-bindings: Add spi-slave property

This is used to indicate that the chip attached to this controller is a SPI
master.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-pxa2xx.txt

@@ -11,6 +11,7 @@ Required properties:
 Optional properties:
 - cs-gpios: list of GPIO chip selects. See the SPI bus bindings,
   Documentation/devicetree/bindings/spi/spi-bus.txt
+- spi-slave: Empty property indicating the SPI controller is used in slave mode.
 
 Child nodes represent devices on the SPI bus
   See ../spi/spi-bus.txt