MAINTAINERS: Make Russell King designated reviewer of phylib

phylink and phylib are interconnected. It makes sense for phylib and
phy driver patches to be also reviewed by the phylink maintainer.
So add Russell King as a designed reviewer of phylib.

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