powerpc/prom: Scan reserved-ranges node for memory reservations

Based on benh's proposal at
https://lists.ozlabs.org/pipermail/linuxppc-dev/2012-September/101237.html,
this change provides support for reserving memory from the
reserved-ranges node at the root of the device tree.

We just call memblock_reserve on these ranges for now.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>