Convert the Smart Mixed-Signal Connectivity (SMSC) LAN911x/912x
Controller Device Tree binding documentation to json-schema.

Document missing properties.
Make "phy-mode" not required, as many DTS files do not have it, and the
Linux drivers falls back to PHY_INTERFACE_MODE_NA.
Correct nodename in example.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>