ARM: shmobile: bockw: add DT reference

This patch adds a sample DT-based Bock-W "reference" implementation.
The use of platform-specific C-code should be avoided
with this configuration as much as possible.

This patch adds new r8a7778_add_dt_devices() which was same stance
of r8a7790_add_dt_devices()

Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>