mmc: exynos dwmmc: check boot mode before init dwmmc

Before this commit, the mmc devices were always registered
in the same order. So dwmmc channel 0 was registered as mmc 0,
channel 1 as mmc 1, etc.
In case of possibility to boot from more then one device,
the CONFIG_SYS_MMC_ENV_DEV should always point to right mmc device.

This can be achieved by init boot device as first, so it will be
always registered as mmc 0. Thanks to this, the 'saveenv' command
will work fine for all mmc boot devices.

Exynos based boards usually uses mmc host channels configuration:
- 0, or 0+1 for 8 bit  - as a default boot device (usually eMMC)
- 2 for 4bit - as an optional boot device (usually SD card slot)

And usually the boot order is defined by OM pin configuration,
which can be changed in a few ways, eg.
- Odroid U3     - eMMC card insertion -> first boot from eMMC
- Odroid X2/XU3 - boot priority jumper

By this commit, Exynos dwmmc driver will check the OM pin configuration,
and then try to init the boot device and register it as mmc 0.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Akshay Saraswat <akshay.s@samsung.com>