staging: mt7621-dts: update ethernet settings.

The ethernet in mt7621 is now supported by
 drivers/net/ethernet/mediatek/

which provides support for the integrated switch through DSA.

This requires some devicetree changes, and particularly allows
a board dts to identify which switch ports are present.

The second CPU interface - gmac1 - doesn't work yet, so the device
tree information may not be correct.  The phy (which is present on the
gnubee-pc2) can negotiate and report connection speed etc, but no
traffic flows.

The gnubee-pc1 has two network ports which are 'black' and 'blue'.
There are connected to switch ports 0 and 4 respectively.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>