linux-guf: Add linux-guf 5.15.29 for imx6(ull). Do not use this by default
The most recent mainline LTS 5.15 kernel is 5.15.29. So use this one for the limux-guf implementation.
The port is based on on a cleaned linux-guf 5.10.104. Commit e55f7fa9b7dd2f069e43eb36411736d90a4d0611.
The kernel can be build using PREFERRED_VERSION_linux-guf = "5.15.29"
in the local.conf