net: phy: broadcom: Add genphy_suspend and genphy_resume for BCM5464

This puts the quad PHY ports in power-down mode when the PHY transitions
to the PHY_HALTED state.  It is likely that all the other PHYs support
the BMCR_PDOWN bit, but I only have the BCM5464R to test.

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