remoteproc: Split firmware name allocation from rproc_alloc()

Make the firmware name allocation a function on its own in an
effort to cleanup function rproc_alloc().

Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20200415204858.2448-3-mathieu.poirier@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>