dt-bindings: arm: Document Renesas Ebisu board DT bindings

This patch adds device tree bindings documentation for Renesas
Ebisu board (RTP0RC77990SEB0010S).

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>