imx: kontron-sl-mx8mm: Remove deprecated phy-mode property

This was previously needed, but U-Boot is now capable of parsing
the new "phy-connection-type" property that is already used in
the main devicetree.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>