[POWERPC] In booting-without-of.txt, clarify that properties must precede subnodes

A strict reading of the flattened device tree format defined in
booting-without-of.txt does in fact require that all the tags defining
properties for a node go before any definitions of subnodes, however
it's not particularly emphasised.  Although allowing intermingled
properties and subnodes would not be ambiguous in meaning, the kernel
parser does currently require that properties precede subnodes.
Furthermore, keeping this constraint makes life easier for various
device tree scanning tools.

Therefore, re-emphasise in booting-without-of.txt that this is a
strict requirement of the flattened device tree format.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>