mediatek: move out-of-tree DTS files to dedicated dts folder

mediatek: move out-of-tree DTS files to dedicated dts folder

Use dedicated dts folder like on ramips to store device tree source
files for boards not already supported in vanilla Linux.
Doing so instead of having them in files-* has several advantages:
 * we don't need to duplicate them for several kernel versions
 * changes to a device tree don't trigger a complete kernel rebuild
 * the files are more obvious to find

Signed-off-by: Daniel Golle <daniel@makrotopia.org>