ARM: shmobile: Document R-Car D3 SoC DT bindings

Note that r8a77995 is the first Renesas "r8a<n>" SoC matching against a 5
digit number, as r8a77990 will be a different SoC.

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>