memory: brcmstb: dpfe: Remove need for dpfe_dev

We can hook sysfs objects to the parent platform device that we are
created from, no need to have a synthetic dpfe_dev just for that. This
incidentally removes the need for having an index, since we are
guaranteed to have an unique path in the sysfs hiearchy.

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