[PATCH] powerpc: Add flattened device tree documentation

The flattened device tree is the only supported way of booting ARCH=powerpc
kernels on non Open Firmware machines.  The documentation for the flattened
tree format and contents has been discussed on mailing lists and lately has
been living in the dtc git tree.  Really, it ought to go in the kernel's
Documentation directory for maximum visibility.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>