edminiv2: switch to SPL

ED Mini V2 is based on Orion 5x which boots at fixed
address 0xFFFF0000 in NOR Flash. Place SPL there, and
switch U-Boot from .bin to .img format, stored in
NOR Flash at 0xFFF90000.

Note: this patch was tested on HW and works, i.e.
it boots U-Boot properly, but SPL console output
currently does not appear, due to GD being trashed
by arch/arm/lib/spl.c. This trashing is soon to be
removed, and then ED Mini V2 SPL console output will
become visible.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>