remoteproc: support default firmware name in rproc_alloc()

If rproc_alloc isn't given a firmware name, look for a default
one using the "rproc-%s-fw" template.

Signed-off-by: Robert Tivy <rtivy@ti.com>
[add commit log, document change, use snprintf, minor style change]
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>