bcm63xx: mark as source-only

bcm63xx: mark as source-only

Maintaining bcm63xx is a nightmare due to the amount of devices and patches
required, since every board requires an individual patch due to the lack of
full device tree compatibility.
Moreover, there are a lot of devices supported on this target which won't work
due to not having enough resources (16M-32M of RAM and/or 4M of flash).
Therefore, any development efforts should be focused on bmips and support for
those devices with enough resources should be added on bmips target.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>