iommu: add IOMMU_EXEC flag for safely allowing XN mappings

Whilst most IOMMU mappings should probably be non-executable, there
may be cases (HSA?) where executable mappings are required.

This patch introduces a new mapping flag, IOMMU_EXEC, to indicate that
the mapping should be mapped as executable.
Signed-off-by: NWill Deacon <will.deacon@arm.com>

include/linux/iommu.h

@@ -24,9 +24,10 @@
 #include <linux/types.h>
 #include <trace/events/iommu.h>
 
-#define IOMMU_READ	(1)
-#define IOMMU_WRITE	(2)
-#define IOMMU_CACHE	(4) /* DMA cache coherency */
+#define IOMMU_READ	(1 << 0)
+#define IOMMU_WRITE	(1 << 1)
+#define IOMMU_CACHE	(1 << 2) /* DMA cache coherency */
+#define IOMMU_EXEC	(1 << 3)
 
 struct iommu_ops;
 struct iommu_group;