Useful to be able to dump the kernels page tables to check permissions
and memory types - derived from arm64's implementation.

Add a debugfs file to check the page tables. To use this the PPC_PTDUMP
config option must be selected.
Signed-off-by: NRashmica Gupta <rashmicy@gmail.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>