remoteproc: Introduce rproc handle accessor for children

In certain circumstances rpmsg devices needs to acquire a handle to the
ancestor remoteproc instance, e.g. to invoke rproc_report_crash() when a
fatal error is detected. Introduce an interface that walks the device
tree in search for a remoteproc instance and return this.

Tested-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>