lantiq: add support for xrx200 firmware depending on soc type

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

Backport of r42182

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42361 3c298f89-4303-0410-b956-a3cf2f4a3e73