ARM: dts: Add dwmmc DT nodes for exynos5420 SOC

This patch adds the mmc device tree node entries for exynos5420 SOC.
Exynos5420 has a different version of DWMMC controller,so a new
compatible string is used to distinguish it from the prior SOC's.

Signed-off-by: Yuvaraj Kumar C D <yuvaraj.cd@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>