net: phy: bcm5481x: MII-Lite activation

net: phy: bcm5481x: MII-Lite activation

Broadcom PHYs featuring the BroadR-Reach two-wire link mode are usually
capable to operate in simplified MII mode, without TXER, RXER, CRS and
COL signals as defined for the MII. The absence of COL signal makes
half-duplex link modes impossible, however, the BroadR-Reach modes are
all full-duplex only.
Depending on the IC encapsulation, there exist MII-Lite-only PHYs such
as bcm54811 in MLP. The PHY itself is hardware-strapped to select among
multiple RGMII and MII-Lite modes, but the MII-Lite mode must be also
activated by software.

Add MII-Lite activation for bcm5481x PHYs.

Signed-off-by: Kamil Horák - 2N <kamilh@axis.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/20250708090140.61355-4-kamilh@axis.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>