of: mdio: export of_mdio_parse_addr

Export of_mdio_parse_addr() which allows parsing a given Ethernet PHY
node MDIO address, verify it is within the allowed range, and return
its value. This is going to be useful for the DSA code which needs to
deal with multiple layers of MDIO buses.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>