arm: mm: init: check for upper PAGE_SHIFT bits in pfn_valid()

If the pfn passed is too big and invalid, the checks in
pfn_valid() will pass because it misses to check overflow.
This will cause that calculated page pointer is overflow and
invalid, then kernel panic happens. Add checks for pfn overflow
in pfn_valid().

Change-Id: I461fb5db8de52f2249b4f70f188d6c5bfe6902f4
Signed-off-by: qqzhou <qqzhou@codeaurora.org>