ARM: shmobile: lager-reference: DTS-only board support

Remove redundant C board code for Lager Multiplatform,
everything is supported via DT these days anyway so it
is fine to rely on the MACHINE_START in setup-r8a7790.c.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
[Remove CONFIG_MACH_LAGER from shmobile_defconfig]
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>