ipq40xx: move Device DTS to dedicated DTS directory

ipq40xx: move Device DTS to dedicated DTS directory

Align the ipq40xx target to the pattern already used on other devices where
the device DTS are placed in a dedicated directory separate from the files
directory.

This, while trying to enforce a common pattern for every target, also permits to
do modification to device DTS without having to trigger a recompilation of the
entire kernel (as the files directory is not touched)

Link: https://github.com/openwrt/openwrt/pull/22040
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>