mmc: mmc-uclass: use the fixed devnum with alias node

If there are alias nodes as "mmc", use the devnum as alias index
number.
This patch is for fixing a problem of Exynos4 series.
Problem is the below thing.

Current legacy mode:
EXYNOS DWMMC: 0, SAMSUNG SDHCI: 1

After using DM:
SAMSUNG SDHCI: 0, EXYNOS DWMMC: 1

Dev index is swapped.
Then u-boot can't find the kernel image..because it is already set to 0 as mmcdev.
If change from legacy to DM, also needs to touch all exynos4 config file.
For using simply, just supporting the fixed devnum with alias node is better than it.

Usage:
alaise {
	....
	mmc0 = &sdhci2; /* eMMC */
	mmc1 = &sdhci1; /* SD */
	...
}

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>