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

Current ARM SMMU probe functions intermingle HW and DT probing
in the initialization functions to detect and programme the ARM SMMU
driver features. In order to allow probing the ARM SMMU with other
firmwares than DT, this patch splits the ARM SMMU 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: Will Deacon <will.deacon@arm.com>
Reviewed-by: Robin Murphy <robin.murphy@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>
Signed-off-by: Will Deacon <will.deacon@arm.com>