net: phy: bcm7xxx: Add support for downshift/Wirespeed

Add support for configuring the downshift/Wirespeed enable/disable
toggles and specify a link retry value ranging from 1 to 9. Since the
integrated BCM7xxx have issues when wirespeed is enabled and EEE is also
enabled, we do disable EEE if wirespeed is enabled.

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