powerpc/kmcent2: update the ethernet devices' phy properties

Change all phy-connection-type properties to phy-mode that are better
supported by the fman driver.

Use the more readable fixed-link node for the 2 sgmii links.

Change the RGMII link to rgmii-id as the clock delays are added by the
phy.

Signed-off-by: Valentin Longchamp <valentin@longchamp.me>
Acked-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: Scott Wood <oss@buserror.net>