drivers: eth_mcux: adding i.mx-rt support

Enables Networking hardware on i.MX-RT type drivers.
Reuses the same eth_mcux driver used by Kinetis family; initialization
sequence refactored to work with this board as well. Unlike Kinetis
family, i.MX has a single ENET interrupt and we need to discriminate
between interrupts using a status register.

Signed-off-by: Andrei Gansari <andrei.gansari@nxp.com>