[PATCH] powerpc: Add support for "linux,usable-memory" on memory nodes

Milton has proposed that we should support a "linux,usable-memory" property
on memory nodes which describes, in preference to "reg", the regions of memory
Linux should use.

This facility is required for kdump to inform the second kernel which memory
it should use.

Signed-off-by: Haren Myneni <haren@us.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>