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.

CRs-Fixed: 959535
Change-Id: I782470a2e4d2a66be17ed2b965ba52b7917592f6
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>