spi/s3c64xx: Add new parameter to cs callback

Since most of the chip-selects are simply going to be like
gpio_set_value, it would do good to have the same callback type
so that it could simply be made to point at gpio_set_value.

Signed-off-by: Jassi Brar <jassi.brar@samsung.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>