ramips: mt7620: simplify DTS properties for GMAC

ramips: mt7620: simplify DTS properties for GMAC

There are only 2 options in the driver
for the function of mt7620 internal switch port 4:

  EPHY mode (RJ-45, internal PHY)
  GMAC mode (RGMII, external PHY)

Let the DTS property be boolean instead of string
where EPHY mode is the default.

Fix how the properties are written
for all DTS that use them,
and add missing nodes where applicable,
and remove useless nodes,
and minor DTS formatting.

Signed-off-by: Michael Pratt <mcpratt@pm.me>