[POWERPC] 86xx: Enable the AC97 interface on 8641D board.

HD interface and AC97 interface share some pins and they are enabled at
the same time, In order to use AC97 interface, we need to disable the HD
interface first.

Signed-off-by:Jason Jin<jason.jin@freescale.com>
Acked-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>