video: s3c-fb: add the bit definitions for VIDCON0_VIDOUT_WB

This patch adds the bit definitions for VIDCON0_VIDOUT_WB.
These definitions are used to support writeback for RGB and
i80 interface. Also, output format of writeback is YUV444.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>