net: enetc: include MAC Merge / FP registers in register dump

These have been useful in debugging various problems related to frame
preemption, so make them available through ethtool --register-dump for
later too.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>