iommu/arm-smmu-v3: Move low-level queue fields out of arm_smmu_queue

In preparation for rewriting the command queue insertion code to use a
new algorithm, introduce a new arm_smmu_ll_queue structure which contains
only the information necessary to perform queue arithmetic for a queue
and will later be extended so that we can perform complex atomic
manipulation on some of the fields.

No functional change.

Tested-by: Ganapatrao Kulkarni <gkulkarni@marvell.com>
Signed-off-by: Will Deacon <will@kernel.org>