ARM: dts: Add SPI CS on exynos5250-snow

Although there is only one choice of chipselect it is necessary to
specify it. The driver cannot claim the gpio otherwise.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Acked-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>