ARM: shmobile: Document DT bindings for Product Register

Add device tree binding documentation for the Product Register (PRR),
which provides product and revision information on most Renesas ARM
SoCs.

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