net: enetc: add loopback support for i.MX95 ENETC PF

net: enetc: add loopback support for i.MX95 ENETC PF

Add internal loopback support for i.MX95 ENETC PF, the default loopback
mode is MAC level loopback, the MAC Tx data is looped back onto the Rx.
The MAC interface runs at a fixed 1:8 ratio of NETC clock in MAC-level
loopback mode, with no dependency on Tx clock.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250506080735.3444381-15-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>