memblock: Add memblock_free/reserve_reserved_regions()

So we can avoid export memblock_reserved_init_regions()
Suggested by Ben.

-v2: use __init_memblock attribute

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>