net: phy: bcm7xxx: define constants for our registers

Define constants for the various registers used in
bcm7xxx_28nm_afe_config_init() to help clarify what this workaround is
about.

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