Documentation: add devicetree docs index file

The device tree infrastructure is being genericized so its documentation
moved out of the PowerPC directory.

Signed-off-by: Rob Landley <rlandley@parallels.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>