ARM: shmobile: Add platform device tree bindings documentation

Add Renesas SH-Mobile, R-Mobile, and R-Car Platform Device Tree Bindings
Documentation, listing supported SoCs and boards.

This allows to use checkpatch to validate DTSes referring to Renesas
shmobile SoCs, and boards containing those SoCs.

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