memory: brcmstb: dpfe: prepare support for multiple API versions

Extend the driver, so it can handle different API versions for
interacting with the DCPU. This is in preparation for the upcoming API
v3.

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