ppc4xx: Do not stop booting on any keypress on dlvision-10g

Use CONFIG_AUTOBOOT_KEYED on dlvision-10g so that booting can only be
stopped with well defined keypresses.

Signed-off-by: Dirk Eibach <eibach@gdsys.de>
Signed-off-by: Stefan Roese <sr@denx.de>