ARM: shmobile: Remove Marzen reference DTS

ARM: shmobile: Remove Marzen reference DTS

Now that the DTS file r8a7779-marzen.dts can be used with
board-marzen.c and board-marzen-reference.c, proceed with removing
r8a7779-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>
(cherry picked from commit 47ad265dd443802ead3d7e1aa3ef60ff1a8da952)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>