ARM: shmobile: lager: Add DT reference

This is sufficient to allow boot of the Lager board with
a console without boards-lager.c compiled into the kernel.
This is an example of a minimal but still useful initialisation
of the board using DT as much as possible.

As such it is the same as the boot of Lager that can be achieved
without a board file. The intention of adding this file
is to facilitate further work to allow board specific devices to be
initialised via DT.

Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>