igep00x0: Falcon mode

Implement spl_start_uboot to let Falcon mode work.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>