powerpc/mm: Define MAX_PHYSMEM_BITS for all 64-bit configs

The recent commit 8bc08689 ("powerpc/mm: Only define
MAX_PHYSMEM_BITS in SPARSEMEM configurations") removed our definition
of MAX_PHYSMEM_BITS when SPARSEMEM is disabled.

This inadvertently broke some 64-bit FLATMEM using configs with eg:

  arch/powerpc/include/asm/book3s/64/mmu-hash.h:584:6: error: "MAX_PHYSMEM_BITS" is not defined, evaluates to 0
   #if (MAX_PHYSMEM_BITS > MAX_EA_BITS_PER_CONTEXT)
        ^~~~~~~~~~~~~~~~

Fix it by making sure we define MAX_PHYSMEM_BITS for all 64-bit
configs regardless of SPARSEMEM.

Fixes: 8bc08689

 ("powerpc/mm: Only define MAX_PHYSMEM_BITS in SPARSEMEM configurations")
Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Reported-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>