mmc: dw_mmc: remove the exynos specific code in dw-mmc.c

dw-mmc.c is the general driver file.
So, remove the exynos specific code at dw-mmc.c.
Instead, exynos specific cod can be move into exynos-dw_mmc.c.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Acked-by: Minkyu Kang <mk7.kang@samsung.com>