remoteproc: Keep local copy of firmware name

Storage of the firmware name was inconsistent, either storing a pointer
to a name stored with unknown ownership, or a variable length tacked
onto the end of the struct proc allocated in rproc_alloc.

In preparation for allowing the firmware of an already allocated struct
rproc to be changed, instead always keep a locally maintained copy of
the firmware name.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>