First of all the reset values of MMC interfaces are differently between
Quad and DualLite/Solo SoM.

                Register VEND_SPEC2(0xC8)
    Quad:       0x0
    DualLite:   0x00000006
    default:    0x00000006    (from i.MX6 Reference Manual)

Furthermore the ROM Code of Quad and DualLite uses the MMC interfaces
differently when it loads the bootloader from that device:

                Register DLL_CTRL(0x60)  Bit 25 FBCLK_SEL (0x48)
    Quad:       0x0                      0
    DualLite:   0x01000021               1

Since the linux kernel and barebox driver doesn't reset all registers,
the MMC interface is in an inconsistent state, which leads to boot
failures for some eMMC devices on the i.MX6 DualLite SoM. The errors
look like:

    mmcblk3: error -110 sending stop command, original cmd response 0x900, card status 0x400900
    mmcblk3: error -84 transferring data, sector 24578, nr 2, cmd response 0x900, card status 0x0
    mmcblk3: retrying using single block read
    mmcblk3: error -84 transferring data, sector 24578, nr 2, cmd response 0x900, card status 0x0
    blk_update_request: I/O error, dev mmcblk3, sector 24578

It's sufficient to reset register DLL_CTRL and bit FBCLK_SEL. Register
VEND_SPEC2 has no effect.

Signed-off-by: Stefan Christ <s.christ@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>