net: phy: introduce PHY_INTERFACE_MODE_REVRMII

The "reverse RMII" protocol name is a personal invention, derived from
"reverse MII".

Just like MII, RMII is an asymmetric protocol in that a PHY behaves
differently than a MAC. In the case of RMII, for example:
- the 50 MHz clock signals are either driven by the MAC or by an
  external oscillator (but never by the PHY).
- the PHY can transmit extra in-band control symbols via RXD[1:0] which
  the MAC is supposed to understand, but a PHY isn't.

The "reverse MII" protocol is not standardized either, except for this
web document:
https://www.eetimes.com/reverse-media-independent-interface-revmii-block-architecture/#

In short, it means that the Ethernet controller speaks the 4-bit data
parallel protocol from the perspective of a PHY (it acts like a PHY).
This might mean that it implements clause 22 compatible registers,
although that is optional - the important bit is that its pins can be
connected to an MII MAC and it will 'just work'.

In this discussion thread:
https://lore.kernel.org/netdev/20210201214515.cx6ivvme2tlquge2@skbuf/

we agreed that it would be an abuse of terms to use the "RevMII" name
for anything than the 4-bit parallel MII protocol. But since all the
same concepts can be applied to the 2-bit Reduced MII protocol as well,
here we are introducing a "Reverse RMII" protocol. This means: "behave
like an RMII PHY".

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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