iommu/arm-smmu: Add context init implementation hook

Allocating and initialising a context for a domain is another point
where certain implementations are known to want special behaviour.
Currently the other half of the Cavium workaround comes into play here,
so let's finish the job to get the whole thing right out of the way.

Change-Id: I2fc1b383a5084d6a92e99002ed12b610e0cf7b04
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Git-commit: ba7e4a08
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: resolved trivial merge conflicts]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>