ipq40xx: split files in 6.1 and 6.6 dedicated directory
Since with recent kernel version DTS moved to a dedicated directory,
it's required to split files to per kernel version to follow kernel
version directory structure.
Also makes use of DEVICE_DTS_DIR to target the correct DTS directory
based on the kernel version.
Signed-off-by: Christian Marangi <[email protected]>