bus: qcom: add EBI2 device tree bindings

This adds device tree bindings for the External Bus Interface 2, EBI2
to the Qualcomm SoC drivers.

Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>