memhp: move DIMM devices into dedicated scope with related common methods

Move DIMM devices from global _SB scope to a new \_SB.MHPC
container along with common methods used by DIMMs:
  MCRS, MRST, MPXM, MOST, MEJ00, MSCN, MTFY

this reduces AML size on 12 * #slots bytes,
i.e. up to 3072 bytes for 265 slots.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>