arm64: dts: m3ulcb: add M3ULCB board DT bindings

Add M3ULCB Device tree bindings Documentation, listing it as a supported
board.

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>