kernel:config: Moved kernel defconfig to correct folder

The defconfig for the 5.15.5 kernel is used by the mx8mm and mx8mp.
The location "imx8mguf" is only accessible by the mx8mm. To make
the config available to the mx8mp, the config folder was renamed.

Note: imx8mguf is the machine name of the mx8mm. This is a known bug
and will be fixed in the future.