imx8m*_venice: move venice to OF_UPSTREAM

Move to imx8m{m,n,p}-venice to OF_UPSTREAM:
 - replace the non-upstream generic imx8m{m,n,p}-venice dt with one of the
   dt's from the OF_LIST
 - handle the fact that dtbs now have a 'freescale/' prefix
 - imply OF_UPSTREAM
 - remove rudundant files from arch/arm/dts leaving only the
   *-u-boot.dtsi files

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>