arm: dts: exynos: Use common alias for Odroid U3/X2 MMC2 (SD-card)

Use MMC0 for eMMC and MMC2 for SD-card as other Exynos-based boards do.
This allows to use common code to get MMC device id based on the XOM[7:5]
pins.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Anand Moon <linux.amoon@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>