This is a series of fairly minor patches that get swiotlb working
on 32-bit powerpc systems with HIGHMEM, plus some cleanup of the
outdated comments in the code. I've made a couple of things
weak that ppc needs to override, and have changed the prototypes
for a couple of functions to include the hwdev pointer, which
we need to ppc to convert bus addresses to and from phys/virt
addresses. I've also fixed a build warning I've been seeing on
ppc.
I have not tested this in any way on any non-ppc platforms,
so commentary/testing from x86/ia64 folks is greatly appreciated.