dt-binding: soc: qcom: Add binding for rmtfs memory

This adds the binding for describing shared memory used to exchange file
system blocks between the RMTFS client and service. A client for this is
generally found in the modem firmware and is used for accessing
persistent storage for things such as radio calibration.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>