iommu/arm-smmu: use mutex instead of spinlock for locking page tables

When creating IO mappings, we lazily allocate our page tables using the
standard, non-atomic allocator functions. This presents us with a
problem, since our page tables are protected with a spinlock.

This patch reworks the smmu_domain lock to use a mutex instead of a
spinlock. iova_to_phys is then reworked so that it only reads the page
tables, and can run in a lockless fashion, leaving the mutex to guard
against concurrent mapping threads.

Cc: <stable@vger.kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>