memory: brcmstb: dpfe: support for deferred firmware download

We add support for deferred downloading of the DPFE firmware. It may be
necessary to do this if the root file system containing the firmware
image is not yet available at the time the driver's probe function is
being called.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>