[XEN][POWERPC] Allocate more memory than RMA for Dom0

[XEN][POWERPC] Allocate more memory than RMA for Dom0

The following patch will only effect Dom0 at the moment and allow it
to be created with memory larger than RMA.  It works by allocating
extents (AKA chunks) of memory of an order specified by the processor.
The extent size should be a value that is capabable of mapping a
"large page" so we use the 970s large page size.  These extents are
tracked by an extents list that simply tracks these page allocations
and what there mapping is.  Other sub-systems effected by this:
  - Dom0 OFD devtree /memory cleanup
  - log_large_page_sizes is now an array
  - Config for minumum Dom0 size

Caveats:
 - There is a hack around HTAB allocation because we currently
   allocate one at domain creation time, this will be correct by a
   patch to follow.
 - Dom0, or domains capable of mapping mfns byt have their extents be
   PFN=MFN

Signed-off-by: Jimi Xenidis <jimix@watson.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>