iommu/arm-smmu: Reorganize code to allow no gdsc

Some SMMUs may not have a dedicated gdsc for it.
Allow for the enable code to prepare clocks and vote
for bus requests even if there is no gdsc provided.

Change-Id: Iaedfb73d8ae7f12895472893468e5c32d2bf2462
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>