MAINTAINERS: Add Andrew Lunn as co-maintainer of PHYLIB

Andrew has been contributing a lot to PHYLIB over the past months and
his feedback on patches is more than welcome.

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