iommu: arm-smmu: Use remote_spin_lock_irqsave() variant

Use of irqsave() locking variants are preferred since they restore
interrupts properly both if either locks are nested or not nested.

Change-Id: I7d794ca9a5e850683403b0e4696e4f317f05cf21
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>