stmmac: dwmac-sti: add PM ops and resume function

Implement PM callbacks and driver remove in the driver instead
of relying on the init/exit hooks in stmmac_platform. This gives
the driver more flexibility in how the code is organized.

Eventually the init/exit callbacks will be deprecated in favor
of the standard PM callbacks and driver remove function.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Tested-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>