spi: dw: Rename "cs-gpio" to "cs-gpios"

This property is named differently than other SPI drivers with the same
property, as well as the property as used in Linux.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Tested-by Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>