[kernel][iommu] Relax const-ness of some accessors

This allows more flexibility in the implementations.

Change-Id: Ie59c4ede6fcc3130e10455a7c38dc0177663188c

[syscalls] Update parameter types and names

This CL updates the parameter types and names for a number of syscalls
to match the Zircon System API rubric. There's more work to do to make
all the syscalls conform to the rubric, but this change is a start.

Change-Id: I218ac5e7e0cbd80a8c69fef7891ad40b78b6b407

[kernel][iommu] Introduce MapContiguous

This will let the BTI/PMT system request contiguous device-physical
addresses for contiguous buffers.

Change-Id: Ic46f808a52d65c8b1cd37bf68563c50f4ff14406

[kernel][iommu] Rework Map API

This introduces the concept of minimum contiguity, which expresses the
IOMMU's capability for creating contiguous device-virtual addreses.

This will allow implementations to more efficiently handle Map requests
according to the guarantees they can provide.

Change-Id: I0fc2f99c9435553a1277b4833a3b39059b8f57fe

[object][iommu] Wire DummyIommu into IommuDispatcher

ZX-693

Change-Id: I54e970b299d646c80712c92ec3f702ec774b0f0d

[dev][iommu] Define an IOMMU API and a no-op implementation

This defines an initial version of the IOMMU API. It is not expressive
enough to handle devices that use PASID support (multiple address spaces
for a single device), but ideally covers all other uses.

ZX-693

Change-Id: I8fbd17a2857e0e8be156c9c59073398a937de107