ARM: shmobile: Let Marzen multiplatform boot with Marzen DTB

Let the multiplatform Marzen support boot with the legacy DTS for
Marzen as well as the Marzen reference DTS.

Based on work for the Koelsch board by Laurent Pinchart.

Cc: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>