net: stmmac: socfpga: Enable internal GMII when using 1000BaseX

Dwmac Socfpga may be used with an instance of a Lynx / Altera TSE PCS,
in which case it gains support for 1000BaseX.

It appears that the PCS is wired to the MAC through an internal GMII
bus. Make sure that we enable the GMII_MII mode for the internal MAC when
using 1000BaseX.

Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250424071223.221239-2-maxime.chevallier@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>