spapr: Clean up RMA size calculation

spapr: Clean up RMA size calculation

Move the calculation of the Real Mode Area (RMA) size into a helper
function.  While we're there clean it up and correct it in a few ways:
  * Add comments making it clearer where the various constraints come from
  * Remove a pointless check that the RMA fits within Node 0 (we've just
    clamped it so that it does)

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>