iommu: ignore scm_call return if SMC_ID not supported

Not all targets support hyp_assign. Ignore the return error if scm_call
return indicate that SMC_ID is not supported.

Change-Id: Ia3e25b1c7d1f4b24f6da0143901aa1cb9a8ea3e2
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>