It's not always useful to pass extra data to the list walker function.
For example, if the list walker function is a class method, state can be
passed around by simply modifying class instance variables. Update
`ListWalker.walk' to accept extra arguments optionally, not require
them.

Change-Id: I433f1e34db5c143c838f0eb2ed7f58203712ccb3

The IOMMU parser currently only supports short-descriptor format page
tables but hardware supports long-descriptor format page tables. Add
support for the long-descriptor format. Most of the existing LPAE MMU
code can be re-used for this, although it must be refactored a bit.

Change-Id: I464d7edeb8818c0be9de1e1a8224bc9b478eac37

The Iommu domain parsing code is common across different Iommu page
table types (normal vs LPAE). Pull this code out so that it can be
easily re-used later when LPAE page table support is added.

Change-Id: Ifc39c12428a9c090f8197d997ddd293e0ae1f0b1