soc: xtensa: imx8: Split generic i.MX8 SoC into i.MX8QXP and i.MX8QM

This commit attempts to split the generic i.MX8 SoC into its
QXP and QM variants. As things are now, the i.MX8 SoC doesn't
have any NXP HAL files to back it up. As a consequence, the
native Zephyr drivers cannot be used.

To solve this issue, the generic i.MX8 has been split into
i.MX8QXP and i.MX8QM, each of them having different NXP HAL
files.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>