net: phy: broadcom: define Broadcom pseudo-PHY address in brcmphy.h

Define the pseudo-PHY address (30) which is used by all Broadcom
Ethernet switches in a shared header file.

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