mmc: exynos_dw_mmc: use the exynos specific data structure

Clksel value is exynos specific value.
It removed "clksel_val" into dwmci_host and created the
"dwmci_exynos_priv_data" structure for exynos specific data.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>