net: enetc: add definition for offset between eMAC and pMAC regs

This is a preliminary patch which replaces the hardcoded 0x1000 present
in other PM1 (port MAC 1, aka pMAC) register definitions, which is an
offset to the PM0 (port MAC 0, aka eMAC) equivalent register.
This definition will be used in more places by future code.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>