arm64: dts: imx8mm-venice-gw7901: fix port/phy validation

Since commit 65ac79e1 ("net: dsa: microchip: add the phylink
get_caps") the phy-mode must be set otherwise the switch driver will
assume "NA" mode and invalidate the port.

Fixes: 65ac79e1 ("net: dsa: microchip: add the phylink get_caps")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>