sperating tx2_smmu from arm_smmu

Signed-off-by: Oliver Scott <Oliver.Scott@data61.csiro.au>

trivial: Fix compiler warnings

- Declare unused variables
- Move variable initialization to prevent uninitialized use
- Correctly store bitwise operation result
- Fixup memset calls to use correct size
- Add missing return statement to functions
- Conditionally define functions that are used conditionally

sel4utils: s/CONFIG_IOMMU/CONFIG_ARM_SMMU

CONFIG_IOMMU does not exist on arm.

vspace: extend map object to io and ept structures

This will allow the same code to be used irrespective of the type of
paging structions used