[PATCH] arm: add comment about max_low_pfn/max_pfn

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From: Russell King <rmk+lkml@arm.linux.org.uk>

Oddly, max_low_pfn/max_pfn end up being the number of pages in the system,
rather than the maximum PFN on ARM.  This doesn't seem to cause any problems,
so just add a note about it.

Signed-off-by: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>