environment: ti: Make get_fdt_mmc common

Since get_fdt_mmc is common, factor it out into mmc.env and remove
it from each platform env file along with changing the directory path to
reflect the standards. Use it in mmcloados but keep loadfdt
defined in case it is still used by some external uEnv.txt script.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>