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

This helps validating DTS files.

Introduced changes:
1. Swapped #address-cells and #size-cells values
2. Renamed node: s/enet-gphy/ethernet-phy@/

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>