powerpc: Fixed memory reserve map layout

The memory reserve map is suppose to be a pair of 64-bit integers
to represent each region.  On ppc32 the code was treating the
pair as two 32-bit integers.  Additional the prom_init code was
producing the wrong layout on ppc32.

Added a simple check to try to provide backwards compatibility.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>