The existing driver supports gpio based /cs signal.
For controller's that have one device per controller,
the slave device's /cs signal might be internally controlled
by the chip select bit of slave select register. They are not
externally asserted/deasserted using gpio pin.

This patch adds support for controllers with dedicated /cs pin.
if "cs-gpio" property doesnt exist in a spi dts node, the controller
would treat the /cs pin as dedicated.
Signed-off-by: NGirish K S <ks.giri@samsung.com>
Signed-off-by: NMark Brown <broonie@linaro.org>