stmmac: review the wake-up support

If the PM support is available this is passed
through the platform instead to be hard-coded
in the core files.
WoL on Magic Frame can be enabled by using
the ethtool support.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>