ramips: mt7620: add ephy-disable option to switch driver

ramips: mt7620: add ephy-disable option to switch driver

Add back the register write to disable internal PHYs
as a separate option in the code that can be set using a DTS property.

Set the option to true by default
when an external mt7530 switch is identified.

This makes the driver more in sync with original SDK code
while keeping the lines separated into different options
to accommodate any board with any PHY layout.

Signed-off-by: Michael Pratt <mcpratt@pm.me>
(cherry picked from commit cc6fd6fbb505071e08011f7998afaffefcf08fd3)