gpio: of: Handle the Freescale SPI CS

The Freescale SPI chipselects are special: while everyone else
is using "cs-gpios" the Freescale platforms just use "gpios".
Fix this by responding with "gpios" when asking for "cs-gpios"
in a freescale device node, so we hide this pecularity from
the SPI core.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>