mmc: Add support for enumerating MMC card in a given mode using mmc command

Add support for enumerating MMC card in a given mode using mmc rescan and
mmc dev commands. The speed mode is provided as the last argument in these
commands and is indicated using the index from enum bus_mode in
include/mmc.h. A speed mode can be set only if it has already been enabled
in the device tree.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>