stmmac: use custom init/exit functions in pm ops

Freeze and restore can call the custom init/exit functions.
Also the patch adds a custom data field that can be used
for storing platform data useful on restore the embedded
setup (e.g. GPIO, SYSCFG).

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