net: stmmac: dwmac-rk: keep the PHY up for WoL

When suspending the machine, do not shutdown the external PHY by cutting
its regulator in the mac platform driver suspend code if Wake-on-Lan is enabled,
else it cannot wake us up.
In order to do this, split the suspend/resume callbacks from the
init/exit callbacks, so we can condition the power-down on the lack of
need to wake-up from the LAN but do it unconditionally when unloading the
module.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>