iommu: msm: add iommu->capable hook

Add iommu->capable hook to legacy iommu driver. Based
upon legacy support it returns false for all present
possible capabilities.

Change-Id: Ia4ee2e01c65a541a7154cb7b4c6ada281da3a200
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>