iommu/arm-smmu: Fix string format warnings

Fix some string format errors that were causing compiler warnings with
gcc 7.4.1.

Fixes: f6081209 ("iommu/arm-smmu: report BID, PID and MID of client on context
 fault")
Change-Id: Ic0dedbadfdc717e4e4769f4075847a7ffa050a1c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>