ARM: shmobile: Let Lager multiplatform boot with Lager DTB

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

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