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>