ARM: DMA-mapping: add missing GFP_DMA flag for atomic buffer allocation

Atomic pool should always be allocated from DMA zone if such zone is
available in the system to avoid issues caused by limited dma mask of
any of the devices used for making an atomic allocation.

Reported-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Stable <stable@vger.kernel.org>	[v3.6+]