net: stmmac: dwmac-rk: add pd_gmac support for rk3399

Add the gmac power domain support for rk3399, in order to save more
power consumption.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: David S. Miller <davem@davemloft.net>