net: phy: add I2C mdio bus

Add an I2C MDIO bus bridge library, to allow phylib to access PHYs which
are connected to an I2C bus instead of the more conventional MDIO bus.
Such PHYs can be found in SFP adapters and SFF modules.

Since PHYs appear at I2C bus address 0x40..0x5f, and 0x50/0x51 are
reserved for SFP EEPROMs/diagnostics, we must not allow the MDIO bus
to access these I2C addresses.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>