memory: brcmstb: dpfe: Fixup API version/commands for 7211

7211 uses a newer version of API v2 which is half way between what was
defined as API v3 and what used to be called API v2 but was used with
DPFE firmwares with major versions 1.x.x.x. Starting with **the new**
API v2, we are no longer getting loadable firmware images, so the
capability to load it is removed (like v3).

To avoid spreading more confusion, map 7268/7271/7278 to the old DPFE
API version 2, 7211 to the new API v2 and introduce the specific
commands for that, and leave newer versions to map to API v3.

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