remoteproc: qcom: Add support for parsing fw dt bindings

Add support for parsing "firmware-name" dt bindings which specifies
the relative paths of mba/modem/pas image as strings. Fallback to
the default paths for mba/modem/pas image on -EINVAL.

Signed-off-by: Sibi Sankar <sibis@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>