dt: Move device tree documentation out of powerpc directory

The device tree is used by more than just PowerPC.  Make the documentation
directory available to all.

v2: reorganized files while moving to create arch and driver specific
    directories.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>
Acked-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>