arm64: imx: imx8mp-debix-model-a: Migrate to OF_UPSTREAM

Device tree for this board can be deleted. Device tree location
now points to the freescale/ directory.

Use absolute path to PMIC node entry and its regulators as
device tree in kernel does not provide corresponding labels

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