verdin-am62: move verdin am62 to OF_UPSTREAM

Move verdin-am62 to OF_UPSTREAM:
- handle the fact that dtbs now have a 'ti/' prefix
- imply OF_UPSTREAM
- remove redundant files from arch/arm/dts leaving only the
  *-u-boot.dtsi files
- update MAINTAINERS file

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>