[PATCH] ppc64: increase MAX_PHYSMEM_BITS to 2PB

* Required for kernel 4.20

With kernel commit 4ffe713b7587 ("powerpc/mm: Increase the max addressable
memory to 2PB"), MAX_PHYSMEM_BITS is bumped up to 51 for SPARSEMEM_VMEMMAP
and SPARSEMEM_EXTREME case. Make the appropriate update here.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>