memory_present() is how each arch/subarch will tell sparsemem and discontigmem
where all of its memory is.  This is what triggers sparse to go out and create
its mappings for the memory, as well as allocate the mem_map[].

By: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>