iommu: msm: Fix unclocked register access from fault handler

Now that non-secure CB faults also gets routed to secure
fault handler (when IOMMU is secure), we need to ensure
that custom fault handler also gets an access with clock
enabled. This was the BUG but no one earlier called
custom fault handler from secure fault handler and remained
uncaught. Fix it by maintaining clock until we return.

Also, ensure that only for valid FSR values we enter and
invoke custom fault handlers.

Change-Id: I9f883c96a8fc05409d5670c35c86301fab6c2f8c
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>