ramips: mt7621-dts: fix claiming rgmii2 pin group for EdgeRouter X SFP

ramips: mt7621-dts: fix claiming rgmii2 pin group for EdgeRouter X SFP

For a TX->TX connected external phy to transmit/receive data, the rgmii2
pin group needs to be claimed with gpio function, at least for EdgeRouter X
SFP. We already claim the pin group under the pinctrl node with gpio
function on the gpio node on mt7621_ubnt_edgerouter-x.dtsi.

However, we should claim a pin group under its consumer node. It's the
ethernet node in this case, which we already claim the rgmii2 pin group
under it on mt7621.dtsi. Therefore, set the function as gpio on the rgmii2
node for EdgeRouter X SFP and get rid of claiming the rgmii2 pin group
under the pinctrl node. With this change, we also get to remove a
definition from mt7621_ubnt_edgerouter-x.dtsi which is specific to
EdgeRouter X SFP.

This change is tested on an EdgeRouter X SFP.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
(cherry picked from commit a1ad8377355c41a7cc3012b707fcecab923cbc90)