ARM: uniphier: select a correct mmc device before flashing images

Some boards support an SD card and an eMMC device at the same time.
Since both belong to 'mmc', they are identified by a device number.
When the device number of the eMMC is 1 instead 0, "mmc dev" command
must be performed to switch the target device before flashing images.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>