memory: brcmstb: dpfe: skip downloading firmware when possible

We want to skip downloading the DPFE firmware from Linux if it was
already downloaded by the boot loader.

The driver now checks if the DCPU is already running and, if so,
whether it can process commands. If the DCPU processes commands
successfully, the driver skips the firmware download step.

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