net: enetc: extract common ENETC PF parts for LS1028A and i.MX95 platforms

The ENETC PF driver of LS1028A (rev 1.0) is incompatible with the version
used on the i.MX95 platform (rev 4.1), except for the station interface
(SI) part. To reduce code redundancy and prepare for a new driver for rev
4.1 and later, extract shared interfaces from enetc_pf.c and move them to
enetc_pf_common.c. This refactoring lays the groundwork for compiling
enetc_pf_common.c into a shared driver for both platforms' PF drivers.

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