iommu/arm-smmu: use a threaded handler for context interrupts

Context interrupts can call domain-specific handlers which might sleep.
Currently we register our handler with request_irq, so our handler is
called in atomic context, so domain handlers that sleep result in an
invalid context BUG.  Fix this by using request_threaded_irq.

This also prepares the way for doing things like enabling clocks within
our interrupt handler.

Change-Id: I2f12f2aa234b62532807a208cd70f2d705083343
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>