ARM: shmobile: Remove Koelsch reference DTS

Now that the DTS file r8a7790-koelsch.dts can be used with
board-koelsch.c and board-koelsch-reference.c, proceed with removing
r8a7790-koelsch-reference.dts.

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