net: of_mdio: factor out code to parse a phy's 'reg' property

Factor out some logic into of_mdio_parse_addr() so it can be reused
later. While at it, use of_property_read_u32() rather than open-coding
the same logic again.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>