net: enetc: detect frame preemption hardware capability

Similar to other TSN features, query the Station Interface capability
register to see whether preemption is supported on this port or not.
On LS1028A, preemption is available on ports 0 and 2, but not on 1
and 3.

This will allow us in the future to write the pMAC registers only on the
ENETC ports where a pMAC actually exists.

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