dts: rt11xx: add enet1g peripheral and set up clock

The enet1g peripheral was missing in device tree for nxp rt11xx.
With this commit, the peripheral can be operated like the enet peripheral
with the eth_mcux (kinetis-ethernet) driver at 10/100 Mbit (no gigabit).

Signed-off-by: Nils Larsen <nils.larsen@posteo.de>