arm: exynos: Read default MMC device from XOM[7:5] pins

XOM pins provide information for iROM bootloader about the boot device.
Those pins are mapped to lower bits of OP_MODE register (0x10000008),
which is common for all Exynos SoC variants. Set the default MMC device id
to reflect the boot device selected by XOM[7:5] pins (2 for the SD or 0 for
the eMMC).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>