boards: nxp: imx8mp: Fix unit and first address mismatch

This fixes the following warning:

> devicetree error: unit address and first address in 'reg' (0x1) don't
> match for /soc/enet@30be0000/mdio/phy@0

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>