spl: nor: Don't allocate header on stack

spl_image_info.name contains a reference to legacy_img_hdr. If we allocate
the latter on the stack, it will be clobbered after we return. This was
addressed for NAND back in 06377c5a ("spl: spl_legacy: Fix NAND boot on
OMAP3 BeagleBoard"), but that commit didn't fix NOR.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>