arm/dt: iommu: Enable alt. core clock for IOMMU

Enable alternative core clock for graphics IOMMU.
Graphics IOMMU requires an extra clock to allow us to
access graphics IOMMU registers. Without this clock we
might end up with un-clocked access when accessing the
IOMMU registers.

Change-Id: I75a4eedeef062c48bca423fbaaee1176b771c4f1
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>