arm: shmobile: Document Kingfisher board DT bindings

Add Kingfisher Device tree bindings Documentation, listing it as a
supported board.
Kingfisher is the H3ULCB/M3ULCB extension board.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>