dt-bindings: Add binding for cros-ec-rpmsg

Add a DT binding documentation for ChromeOS EC driver over rpmsg.

Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>