stmmac: dwmac-rk: turn resume/suspend into standard PM callbacks

Use standard PM resume/suspend callbacks instead of the hooks in
stmmac_platform. This gives the driver more control and flexibility
when implementing PM functionality. The hooks in stmmac_platform
also doesn't buy us anything extra.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>