iommu: Make termination of stalled transaction optional

When pagefault happens do not cancel the faulting transaction if
the registered fault handler returns EBUSY error. This way
drivers can control when they want to resume the transaction.

Change-Id: Ia4563da073ab04174803101c3b8ec82b0571850e
Signed-off-by: Shubhraprakash Das <sadas@codeaurora.org>