[ARM] mmu: add option to map lowmem with page mappings

[ARM] mmu: add option to map lowmem with page mappings

add a kernel configuration to map the kernel's lowmem pages using PTE
mappings, rather than the default behavior of 1MiB section mappings.
on ARMv7 processors, to support allocating pages with DMA-coherent
cache attributes, the cache attributes specified in the kernel's
mapping must match cache attributes specified for other mappings;
to ensure that this is the case, the kernel's attributes must be
specified on a per-page basis.

to avoid problems caused by the init_mm page table allocations exceeding
the available initial memory, when this config is enabled lowmem is
initially mapped using sections (matches current behavior), then remapped
using pages after bootmem is initialized

Change-Id: I8a6feba1d6806d007e17d9d4616525b0446c0fb1
Signed-off-by: Gary King <gking@nvidia.com>