iommu/arm-smmu-v3: Add missing S2FWB feature detection

iommu/arm-smmu-v3: Add missing S2FWB feature detection

Commit 67e4fe398513 ("iommu/arm-smmu-v3: Use S2FWB for NESTED domains")
introduced S2FWB usage but omitted the corresponding feature detection.
As a result, vIOMMU allocation fails on FVP in arm_vsmmu_alloc(), due to
the following check:

	if (!arm_smmu_master_canwbs(master) &&
	    !(smmu->features & ARM_SMMU_FEAT_S2FWB))
		return ERR_PTR(-EOPNOTSUPP);

This patch adds the missing detection logic to prevent allocation
failure when S2FWB is supported.

Fixes: 67e4fe398513 ("iommu/arm-smmu-v3: Use S2FWB for NESTED domains")
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Link: https://lore.kernel.org/r/20250408033351.1012411-1-aneesh.kumar@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>