net: enetc: build enetc_pf_common.c as a separate module

Compile enetc_pf_common.c as a standalone module to allow shared usage
between ENETC v1 and v4 PF drivers. Add struct enetc_pf_ops to register
different hardware operation interfaces for both ENETC v1 and v4 PF
drivers.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>