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

Implement the needed PM callbacks 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>
Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>