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>