arm: dts: imx8mm-venice-gw7901: add dsa phy handles to u-boot dtsi

The upstream Linux DSA drivers do not require phy-handle nodes in
the DSA ports yet the U-Boot DSA drivers do. Add a phy-handle and
the mdio nodes to the u-boot.dtsi file so that future dts file
syncrhonization between Linux and U-Boot don't break networking.

Fixes: 24a7a3c1 ("imx8mm: synchronise device tree with linux")
Signed-off-by: Tim Harvey <tharvey@gateworks.com>