net: mtk_eth_soc: remove support for RMII and REVMII modes

net: mtk_eth_soc: remove support for RMII and REVMII modes

Since the conversion of mtk_eth_soc to phylink's supported_interfaces
bitmap, these two modes have not been selectable. No one has raised
this as an issue. Checking the in-kernel DT files, none of them use
either of these modes with this hardware.

Daniel Golle concurs:

 A quick grep through the device trees of the more than 650 ramips and
 mediatek boards we support in OpenWrt has revealed that *none* of them
 uses either reduced-MII or reverse-MII PHY modes. I could imaging that
 some more specialized ramips boards may use the RMII 100M PHY mode to
 connect with exotic PHYs for industrial or automotive applications
 (think: for 100BASE-T1 PHY connected via RMII). I have never seen or
 touched such boards, but there are hints that they do exist.

 For reverse-MII there are cases in which the Ralink SoC (Rt305x, for
 example) is used in iNIC mode, ie. connected as a PHY to another SoC,
 and running only a minimal firmware rather than running Linux. Due to
 the lack of external DRAM for the Ralink SoC on this kind of boards,
 the Ralink SoC there will anyway never be able to boot Linux.
 I've seen this e.g. in multimedia devices like early WiFi-connected
 not-yet-so-smart TVs.

Consequently, the conclusion is that no one uses these modes with this
hardware, so we might as well drop support for them.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>