spi: pxa2xx: dt-bindings: Add ready GPIO signal

This this is used to let the SPI master know that our FIFO is filled and
we're ready to service a transfer. Only useful in slave mode.

A signal like this is used by an embedded controller on a OLPC XO 1.75
machine, that happens to be a SPI master.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
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

@@ -12,6 +12,8 @@ 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.
+- ready-gpios: GPIO used to signal a SPI master that the FIFO is filled
+  and we're ready to service a transfer. Only useful in slave mode.
 
 Child nodes represent devices on the SPI bus
   See ../spi/spi-bus.txt