cg3: switch to load_image_mr() and remove prom-addr hack

Previous to the existence of load_image_mr(), the only way to load in the
FCode ROM image was to pass in its physical address via qdev properties
and use load_image_targphys().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
