ARM: uniphier: add a command to find the first MMC (non-SD) device

UniPhier SoC family supports both (e)MMC boot and SD card boot;
however, both of them are handled in the same uclass.

When booting from the eMMC, we want to know the device number
of the (e)MMC, not SD.  This command is useful to find the first
MMC (non-SD) device.

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