net: samples: Add mimxrt1020_evk overlay for OT's RCP Host support (UART)

This overlay configures the imxrt1020 EVK board to use its lpuart2 serial
port to communicate with other board (in this case nRF21540-DK), which
is the RCP for OpenThread protocol.

The nRF Zephyr RCP is the one from Nordic (v2.7.0):
west build -p always -b nrf21540dk/nrf52840 -S logging
		nrf/samples/openthread/coprocessor

The same UART (on nRF) can be used for testing/debugging purposes with
using the ot-cli's SPINEL communication program:
sudo ./build/posix/src/posix/ot-cli 'spinel+hdlc+uart:///dev/ttyACM1?u


art-baudrate=1000000' --verbose

Signed-off-by: Lukasz Majewski <lukma@denx.de>