iommu/arm-smmu: Add implementation specific CB initialization hook

Some SMMU implementations, such as the QSMMUV500 model, may
need to program a specific context bank, in addition to the
initial context bank programming. Thus, add support for
implementation specific context bank initialization, and
migrate the QSMMUV500 context bank initialization to use
the new hook.

Consequently, as there are no current users of the
arm_smmu_arch_init_context_bank() hook, remove it.

Change-Id: Ia89420cc8b669a142b37bf4250c09b6aa3646172
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>