cmd_mmc.c: Add bootbus mmc sub-command

Add a bootbus sub-command to the mmc command to allow for setting
the boot_bus_width, reset_boot_bus_width and boot_mode fields of
BOOT_BUS_WIDTH (EXT_CSD[177]).

Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Tom Rini <trini@ti.com>
Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>