net: bcmgenet: implement GPHY power down sequence

Implement the GPHY power down sequence by setting all power down bits, putting
the GPHY in reset, and finally cutting the 25Mhz reference clock.

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