powerpc/mm: Dump hash table

Useful to be able to dump the kernel hash page table to check
which pages are hashed along with their sizes and other details.

Add a debugfs file to check the hash page table. If radix is enabled
(and so there is no hash page table) then this file doesn't exist. To
use this the PPC_PTDUMP config option must be selected.

Signed-off-by: Rashmica Gupta <rashmicy@gmail.com>
[mpe: Fix build with SPARSEMEM_VMEMMAP=n & PSERIES=n]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>