iommu/arm-smmu: Add NULL check for platform_get_resource return value

platform_get_resource return value 'res' can be NULL. NULL check
platform_get_resource return value before derefereing.

Change-Id: Ifba91104428c5279fad21f892840c67ce4258aee
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>