octeon: re-enable CONFIG_CAVIUM_CN63XXP1 and EdgeRouter image

octeon: re-enable CONFIG_CAVIUM_CN63XXP1 and EdgeRouter image

The symbol CONFIG_CAVIUM_CN63XXP1 was disabled during the bump to
4.19 (see Fixes:) with the following reason:

  No supported hardware uses CN63XXP1 and it causes "slight decrease
  in performance"

However, it later turned out that the edgerouter image needed it,
which led to having the device disabled in [1].
Still, dropping support of a device seems a harsh action for just
removing a "slight" decrease in performance from the other devices.

Thus, this enables CONFIG_CAVIUM_CN63XXP1 again, and essentially
restores the situation present until (including) kernel 4.14 on
this target.

For OpenWrt as a platform, it seems more desirable to support all
devices (and have them tested regularly via the snapshots) in this
case.
Users interested in maximum performance might still just remove
the symbol again in their local build.

[1] 3824fa26d256 ("octeon: disable edgerouter image")

Fixes: 6c22545225cd ("target/octeon: Add Linux 4.19 support")
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit cfd1a40583d66d5f6a30dec5bee0f7ebd4d252ef)