The members here are being used for both the linear and the 2 level case,
with the meaning of each item slightly different in the two cases.

Split it into a clean union where both cases have their own struct with
their own logical names and correct types.

Adjust all the users to detect linear/2lvl and use the right sub structure
and types consistently.

Remove CTXDESC_CD_DWORDS by changing the last places to use
sizeof(struct arm_smmu_cd).
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/8-v4-6416877274e1+1af-smmuv3_tidy_jgg@nvidia.comSigned-off-by: Will Deacon <will@kernel.org>