ARM: S3C64XX: Use clk_prepare_enable/clk_disable_unprepare in dma.c

This patch modifies s3c64xx DMA driver to prepare and unprepare clocks
in addition to enableind and disabling, since it is required by common
clock framework.

Signed-off-by: Tomasz Figa <tomasz.figa@gmail.com>
Tested-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>