arm: dma mapping: Export a dma ops function arm_dma_set_mask

Expose another DMA operations function: arm_dma_set_mask. This
function will be added to a custom DMA ops for Armada 370/XP.
Depending of its configuration Armada 370/XP can be set as a "nearly"
coherent architecture. In this case the DMA ops is made of:
- specific functions for this architecture
- already exposed arm DMA related functions
- the arm_dma_set_mask which was not exposed yet.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>