powerpc/prom: Use DRCONF flags while processing detected LMBs

Replace hard coded values with existing DRCONF flags while procesing
detected LMBs from the device tree. Does not change any functionality.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>