arm: dma mapping: export arm iommu functions

This patch adds EXPORT_SYMBOL_GPL calls to the three arm iommu
functions - arm_iommu_create_mapping, arm_iommu_free_mapping
and arm_iommu_attach_device. These three functions are arm specific
wrapper functions for creating/freeing/using an iommu mapping and
they are called by various drivers. If any of these drivers need
to be built as dynamic modules, these functions need to be exported.

Changelog v2: using EXPORT_SYMBOL_GPL as suggested by Marek.

Signed-off-by: Prathyush K <prathyush.k@samsung.com>
[m.szyprowski: extended with recently introduced
 EXPORT_SYMBOL_GPL(arm_iommu_detach_device)]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>