iommu/amd: remove compiler warning due to IOMMU_CAP_NOEXEC

Some versions of GCC get unduly upset when confronted with a switch
that doesn't explicitly handle all cases of an enum, despite having an
implicit default case following the actualy switch statement:

   drivers/iommu/amd_iommu.c: In function 'amd_iommu_capable':
>> drivers/iommu/amd_iommu.c:3409:2: warning: enumeration value 'IOMMU_CAP_NOEXEC' not handled in switch [-Wswitch]
     switch (cap) {

This patch adds a case for IOMMU_CAP_NOEXEC to the amd IOMMU driver to
remove this warning.

Cc: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>

drivers/iommu/amd_iommu.c

@@ -3411,6 +3411,8 @@ static bool amd_iommu_capable(enum iommu_cap cap)
 		return true;
 	case IOMMU_CAP_INTR_REMAP:
 		return (irq_remapping_enabled == 1);
+	case IOMMU_CAP_NOEXEC:
+		return false;
 	}
 
 	return false;