Merge tag 'u-boot-imx-20220422' of https://gitlab.denx.de/u-boot/custodians/u-boot-imx

u-boot-imx-20220422
-------------------

- Switch to DM_SERIAL
- Drop MMCROOT
- several cleanup

CI: https://source.denx.de/u-boot/custodians/u-boot-imx/-/pipelines/11815