net: phy: bcm7xxx: Modified to use global core register defines

Modified the bcm7xxx phy driver to remove local core register
defines and use the common ones from "include/linux/brcmphy.h"

Signed-off-by: Arun Parameswaran <arunp@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>