Currently the
driver, drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c, and its
bindings, Documentation/devicetree/bindings/net/altr,socfpga-stmmac.yaml,
allow all rgmii phy-modes.

There is nothing unusual about allowing all four RGMII modes. Somebody
might design a board with the 2ns delay via extra long clocks lines,
or some other form of delay lines. Such a board would need 'rgmii',
and they do exist, a board like this was added this cycle. There is
one rather odd board with a DT file which has extra long clock line
for TX, but not RX! That needs rgmii-rxid. The majority of boards
don't have any delays on the PCB, so need 'rgmii-id'.

	Andrew