ARM: shmobile: Add silk device tree bindings documentation

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

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

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>