MAINTAINERS: Add myself as third phylib maintainer

Add myself as third phylib maintainer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>