arm64: add basic DTS for i.MX8MQ

This adds the basic DTS for the i.MX8MQ.
For now only the following peripherals are supported:
- IOMUXC (pin controller)
- CCM (clock controller)
- GPIO
- UART
- uSDHC (SD/eMMC controller)
- FEC (ethernet controller)
- i2c

This is enough to get a very basic board support up and running.

One known limitation is that the driver for the GPC interrupt
controller is still missing, rendering the CPU sleep states unusable
as there is nothing waking them up anymore.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Dong Aisheng <Aisheng.dong@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>