arm: Init SPARSEMEM section for removed memory

If a memblock has been removed with memblock_remove, it will
be skipped in 'for_each_memblock'. If a SPARSEMEM section
is completely enclosed within this removed memblock,
memory_present will never be called and the section will
never be initialized. This will cause garbage dereferences
later on.

This change loops on the memory banks, instead of the memblocks.
Memory banks always exist, regardless of memblock_remove and
ensure that all SPARSEMEM sections will be initialized, even
if they are removed later.