powerpc/powernv: Rename alloc_m64_pe() to reserve_m64_pe()

The patch renames alloc_m64_pe() to reserve_m64_pe() to reflect
its real usage: We reserve PE numbers for M64 segments in advance
and then pick up the reserved PE numbers when building the mapping
between PE numbers and M64 segments.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>