powerpc/44x: boot wrapper: allow kernel to load into non-zero address

For AMP, different kernel instances load into separate memory regions.
Read the start of memory from the device tree and limit the memory to what's
specified in the device tree.

Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>