machine:imx8mguf: Add receipes for tanaro 5.15 kernel.

* Name kernel recipe linux-imx
  Don't use linux-imx-kuk to allow two different entries in the
  SRCREV.conf to specify the SRCREV independently for the 5.10 and
  the 5.15 version. As the 'kuk' part is not in this branch it also
  makes sense to remove the appendix.
* defconfig from 5.10.9 after 'savedefconfig' use with 5.15 and
  savedefconfig from 5.15 kernel again
* Recipe pointing to new kernel branch
* Change the naming of the installed devicetrees with 5.15 kernel
  The subfolder and prefix is now seconorth instead of keithkoep and guf.

BCS 746-000672