msm: iommu: add some more convenience domain-finding functions

Some common usage patterns have emerged among users of the MSM IOMMU
driver:

    -> find a group by group name
      `--> find a domain by group

and:

    -> find a group by group name
      `--> find a domain by group
          `--> find domain number by domain

Add some helper functions to make those use cases more convenient and
slightly less error prone.

Change-Id: Iad5f315436d600d960ab900de0beae7bd3904257
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>