memblock: Add memblock_find_in_range()

This is a wrapper for memblock_find_base() using slightly different
arguments (start,end instead of start,size for example) in order to
make it easier to convert existing arch/x86 code.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>