ARM: shmobile: Let Koelsch multiplatform boot with Koelsch DTB

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

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