arm64: dts: imx8mp-debix: remove unused fec pinctrl node

The SoM A uses the EQOS ethernet interface and not the FEC, so drop the
interface pinctrl node from the device tree.

Fixes: c86d350a ("arm64: dts: Add device tree for the Debix Model A Board")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>