net: mdio: add 2.5g and 5g related PMA speed constants

Add constants indicating 2.5g and 5g ability in the MMD PMA speed
register.

Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/98e15038-d96c-442f-93e4-410100d27866@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>