dt-bindings: net: sms911x: Convert to json-schema

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: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>