dt-bindings: net: generalize moxart-mac to support all faraday based ftmac IP

Andestech and moxa use the same faraday mac IP, so we use faraday,ftmac.txt to
describe it.

Signed-off-by: Greentime Hu <green.hu@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>