[POWERPC] Update documentation for flat device tree format v17

This patch updates booting-without-of.txt to describe version 17 of
the flattened device tree format.  Version 17 is a small, backwards
compatible change from version 16, adding an extra field giving the
size of the device tree's structure block.  At this time, the kernel
has no use for the extra information, however its presence can make
life easier for bootloaders or other software manipulating the tree.

In addition this patch adds information on the size_dt_strings field
of the device tree header, present since version 3 of the flattened
tree format, but omitted from the documentation.  It also makes
changes to consistently refer to versions 16 and 17 as versions 16 and
17 in decimal, rather than version 0x10 which was occasionally used
for version 16 previously.

Finally, we also add the new field to the definition of the device
tree header structure in prom.h

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>