memblock: Make memblock_find_region() out of memblock_alloc_region()

This function will be used to locate a free area to put the new memblock
arrays when attempting to resize them. memblock_alloc_region() is gone,
the two callsites now call memblock_add_region().

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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v2. Fix membase_alloc_nid_region() conversion