MAINTAINERS: Add entry for Broadcom Ethernet PHY drivers

Add an entry for the Broadcom Ethernet PHY drivers covering the BCM63xx,
BCM7xxx, BCM87xx, BCM54140, BCM84881, the venerable broadcom.c driver
and the companion library files.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>