mmc: dw_mmc: exynos: Add a new compatible string for exynos5420

The Exynos5420 has a DWMMC controller which is different from prior
versions.This patch adds a new compatible string for Exynos5420.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>