ARM: shmobile: lager: Remove legacy board support

Lager legacy support level is same as the DT case
so remove the legacy code and force people to move
over to using Multiplatform and DT.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
[Remove lager_defconfig and don't build the dtb for legacy kernels]
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>