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>