memory: brcmstb_dpfe: Allow building Broadcom STB DPFE as module

Allow building the Broadcom STB DPFE driver as a module, it is already a
platform driver proper with all of the resource releasing device
managed.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Markus Mayer <mmayer@broadcom.com>
Link: https://lore.kernel.org/r/20210924031459.8911-1-f.fainelli@gmail.com


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>