ARM: dts: rename mmc dts node for exynos5 series

This patch rename's the device tree mmc node's from "dwmmc" to "mmc".
According to ePAPR chapter 2.2.2 generic node name recommendation,
it has been opted change from dwmmc to mmc.Also this patch remove the
instance index from the node name.

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