ARM: S3C64XX: Use S3C64XX_SDMA_SEL register name instead of numeric address

This patch adds a definition of S3C64XX_SDMA_SEL register and modifies
the s3c64xx DMA driver to use it instead of a magic register address.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>