spl: net: Add FIT image support over network boot

FIT support in the net boot case is much like the RAM boot case in that
we load our image to "load_addr" and pass a dummy read function into
"spl_load_simple_fit()". As the load address is no longer hard-coded to
the final execution address, legacy image loading will require load_addr
to be set correctly in the image header.

Signed-off-by: Andrew F. Davis <afd@ti.com>