ramips: remove superfluous & confusing DT binding

ramips: remove superfluous & confusing DT binding

Mediatek has a reference platform that pairs an MT7620A with an MT7530W,
where the latter responds on MDIO address 0x1f while both chips respond on
0x0 to 0x4. The driver special-cases this arrangement to make sure it's
talking to the right chip, but two different ways in two different places.
This patch consolidates the detection without the current requirement of
both tests to be separately satisfied in the DTS.

Signed-off-by: Daniel Gimpelevich <daniel@gimpelevich.san-francisco.ca.us>