configs/nitrogen8m: Add new defconfig

NXP i.MX8MQ based SBC with 2GB of LPDDR4 and 8GB eMMC.

More details on the platform here:
https://boundarydevices.com/product/nitrogen8m



Signed-off-by: Gary Bisson <gary.bisson@boundarydevices.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>