iommu/arm-smmu: add option to enable static context bank allocation

To implement slave side protection, programming of global registers as
well as secure context bank registers is handed over to TZ. Now, instead
of dynamically allocating context banks, TZ allocates CBs once in pre
defined static manner during boot and this allocation is maintained
throughout the life of system.

Add an option to enable use of this pre-defined context bank allocation.
We would be reading through SMR and S2CR registers at run time to
identify CB allocated for a particular sid.

Change-Id: If0b86bd89b0422c2170551de0d2f6405a79fc255
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>