ti_armv7_common: env: Add support for loading FIT images

FIT is a new image format which is a Tree like structure and gives more
flexibility in handling of various images. Mainly used for unification of
multiple images in a single blob and provide security information for each
image.

U-Boot already has support for loading such images, so adding the environment
support to load FIT image on all TI platforms.

Reviewed-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>