arm64: add support to dump the kernel page tables

In a similar manner to arm, it's useful to be able to dump the page
tables to verify permissions and memory types. Add a debugfs file
to check the page tables.

Change-Id: I319c30c27a85f1686108f3e06ea20eb686c53d09
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>