net: phy: add PHY_INTERFACE_MODE_10GBASER

Recent discussion has revealed that the use of PHY_INTERFACE_MODE_10GKR
is incorrect. Add a 10GBASE-R definition, document both the -R and -KR
versions, and the fact that 10GKR was used incorrectly.

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