spi/s3c64xx: Put the /CS GPIO into output mode

No call was being made by the GPIO driver to put the GPIO into output
mode meaning that the calls to gpio_set_value() which were being done
were not valid. A similar issue appears to exist with the DT GPIO
requests but as they appear to be being used for pinmux it's less clear
to me that we want to configure them.

Without this fix Cragganmore systems can't talk to their SPI devices.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>