dt-bindings: net: brcm,unimac-mdio: convert to the json-schema

This helps validating DTS files.

Introduced example binding changes:
1. Fixed reg formatting
2. Swapped #address-cells and #size-cells incorrect values
3. Renamed node: s/phy/ethernet-phy/

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>