powerpc: Dynamic DMA zone limits

Platform code can call limit_zone_pfn() to set appropriate limits
for ZONE_DMA and ZONE_DMA32, and dma_direct_alloc_coherent() will
select a suitable zone based on a device's mask and the pfn limits that
platform code has configured.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: Shaohui Xie <Shaohui.Xie@freescale.com>

diff --cc arch/powerpc/Kconfig
Simple merge

diff --cc arch/powerpc/include/asm/pgtable.h
Simple merge

diff --cc arch/powerpc/kernel/dma.c
Simple merge

diff --cc arch/powerpc/mm/mem.c
Simple merge