spi/dt: DT bindings documentation: "num-cs" property for SPI controllers

Several SPI controller drivers have defined differently named properties for
the number of chip selects.  Now adding "num-cs" as a reference name for new
bindings.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>