mmc: dw_mmc: exynos: Add support for exynos7

The Exynos7 has a DWMMC controller (v2.70a) which is different from
prior versions. This patch adds new compatible strings for exynos7.
This patch also fixes the CLKSEL register offset on exynos7.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@samsung.com>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>