ARM: mm: restrict early_alloc_aligned to legal area

When early_alloc_aligned() is called, it appears that only memory in the
first memory bank is mapped for CPU access. However, memblock_alloc() is
called to allocate the RAM, and that can return RAM that is part of any
valid memory bank, which hence could be inaccessible to the CPU. If this
happens, the subsequent memset() will hang or crash.

Solve this by calling memblock_alloc_base() instead of memblock_alloc().
This function takes an explicit max address. Use the end of the first
memory bank as the address.

As an example, this issue can be triggered as follows:

* Total of 512MB system RAM, so it is all lowmem not highmem. Without this,
  subsequent banks may be ignored by map_lowmem() due to being highmem.
* RAM is split into multiple banks, due to some RAM somewhere in the
  middle having been allocated for purposes other than Linux, e.g. an LCD
  frame-buffer of for a co-processor.
* Some bank is not section-aligned, so that alloc_init_pte() is called
  rather than __map_init_section().

Signed-off-by: Stephen Warren <swarren@nvidia.com>