imx: imx8mp: Add support for Polyhex Debix Model A SBC

Add support for the Polyhex Debix Model A SBC board.
It is an industrial grade single board computer based on
NXP's i.MX 8M Plus.
Currently supported interfaces are:
- Serial console
- Micro SD
- eQOS and FEC Ethernet

imx8mp-debix-model-a.dts is taken from Linux 6.3.

Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>