boards: mr_canhubk3: enable EMAC

This board has a single instance of EMAC connected to a NXP TJA1103
Ethernet PHY. Currently, there is no driver for this PHY and its
pin strapping configuration allows to use it without software
configuration, so EMAC is configured as fixed link.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>