doc: phy: document some PHY_INTERFACE_MODE_xxx settings

There seems to be some confusion surrounding three PHY interface modes,
specifically 1000BASE-X, 2500BASE-X and SGMII.  Add some documentation
to phylib detailing precisely what these interface modes refer to.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>