iommu/arm-smmu-v3: Split probe functions into DT/generic portions

Current ARM SMMUv3 probe functions intermingle HW and DT probing in the
initialization functions to detect and programme the ARM SMMU v3 driver
features. In order to allow probing the ARM SMMUv3 with other firmwares
than DT, this patch splits the ARM SMMUv3 init functions into DT and HW
specific portions so that other FW interfaces (ie ACPI) can reuse the HW
probing functions and skip the DT portion accordingly.

This patch implements no functional change, only code reshuffling.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Tomasz Nowicki <tn@semihalf.com>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Tested-by: Tomasz Nowicki <tn@semihalf.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Joerg Roedel <joro@8bytes.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>