dt-bindings: arm: Document R-Car E3 SoC DT bindings

This patch adds device tree bindings documentation for Renesas R-Car
E3 (r8a77990).

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>