zephyr: change SoC overlay name from nxp_imx8 to nxp_imx_adsp

Since the i.MX8 SoC will be split into 2 different SoCs:
mimx8qm6 and mimx8qx6, the nxp_imx8 name can no longer be
used. This is because the new SoCs will have different CONFIG_SOC
values such that CONFIG_SOC != nxp_imx8. Despite this, we can take
advantage of the fact that the 2 SoCs use the same toolchain.
As such, rename nxp_imx8 to nxp_imx_adsp. This works because
Zephyr looks for an overlay with the same name as the toolchain
if one with the same name as the SoC doesn't exist.

This won't break the current state of things because the i.MX8
SoC will also fallback to the nxp_imx_adsp overlay.

Signed-off-by: Laurentiu Mihalcea <[email protected]>