dt-binding: remoteproc: Introduce ADSP loader binding

This document defines the binding for a component that loads firmware
and control the life cycle of the Qualcomm ADSP Hexagon core.

Acked-by: Rob Herring <robh@kernel.org>
Cc: Sarangdhar Joshi <spjoshi@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>