iommu: add support for multiple iommu drivers

Some targets may have more than 1 SMMU IPs which
require different driver supports. By default,
all the driver register themselves onto platform
bus. But platform bus cannot support more than
one IOMMU ops implementation. So, move MSM driver
onto separate bus and provide and interface to
select the right bus just before requesting for
a domain.

Add following API:
msm_iommu_get_bus(struct device *dev)

Change-Id: I116ecb203b73157bf5cd6220391344a01a165bb1
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>