Add prom_claim_chunk_top

Add prom_claim_chunk_top

We want temporary allocations to be taken from the top of our address
space so the kernel and initrd can be loaded as low as possible. The
very early kernel code uses the top of the initrd as the low watermark
for memory allocations so the lower this is the better.

We currently see a number of fails where a large initrd causes us
to run out of space in a 128MB RMO region. Allocating the temporary
areas up high and therefore the initrd lower fixes it.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>