arm64: dts: imx8mq: add Nitrogen8 SoM

This adds the description of the Nitrogen8 System on Module. The module
is quite simple with only a few (almost) fixed regulators and a eMMC
on-board.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>