ARM64: mm: pageattr: support CONFIG_DEBUG_PAGEALLOC

ARM64: mm: pageattr: support CONFIG_DEBUG_PAGEALLOC

When CONFIG_DEBUG_PAGEALLOC is enabled, kernel_map_pages
is used by page allocator to map/unmap pages dynamically
into the kernel logical address space. Any access to an
unmapped page is invalid and can help catch memory corruptions.
So, add this function and also related Kconfig definition.

Bug 1492689

Change-Id: Ib61773542f0ec88cf0870348b9251ceff29d5139
Signed-off-by: Sri Krishna chowdary <schowdary@nvidia.com>
Reviewed-on: http://git-master/r/772741
Reviewed-by: Vandana Salve <vsalve@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>