iommu/iommu-debug: Add COHERENT_HTW_DISABLE to attach info

It can be useful to check whether or not coherent hardware table walking
has been explicitly disabled on attached domains.  Add this to the
attach info debugfs file.

Change-Id: I432303ecb734d32eaa02038694daad0d8c4d8aba
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>