arm: make SWIOTLB available

IOMMU_HELPER is needed because SWIOTLB calls iommu_is_span_boundary,
provided by lib/iommu_helper.c.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: will.deacon@arm.com
CC: linux@arm.linux.org.uk


Changes in v8:
- use __phys_to_pfn and __pfn_to_phys.

Changes in v7:
- dma_mark_clean: empty implementation;
- in dma_capable use coherent_dma_mask if dma_mask hasn't been
  allocated.

Changes in v6:
- check for dev->dma_mask being NULL in dma_capable.

Changes in v5:
- implement dma_mark_clean using dmac_flush_range.

Changes in v3:
- dma_capable: do not treat dma_mask as a limit;
- remove SWIOTLB dependency on NEED_SG_DMA_LENGTH.
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 1ad6fb6..b08374f 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1872,6 +1872,12 @@
 	  neutralized via a kernel panic.
 	  This feature requires gcc version 4.2 or above.
 
+config SWIOTLB
+	def_bool y
+
+config IOMMU_HELPER
+	def_bool SWIOTLB
+
 config XEN_DOM0
 	def_bool y
 	depends on XEN