ARM: shmobile: add Porter board DT bindings

Add Porter device tree bindings documentation, listing it as a supported board.

This allows to use checkpatch to validate DTSes referring to the Porter board.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>