dma_mapping_ops for i386

[Jeremy Fitzhardinge <jeremy@goop.org>]

I'm looking at adding dom0 support to the pv-ops kernel.  One of the 
obvious things we need is to support real device drivers, and the 
associated p->m translations for devices.

I'm thinking the cleanest thing to do is make x86-64's dma-mapping.h 
with its dma_mapping_ops common to i386 and x86-64, so we can hook the 
Xen translations in there.  Presumably we'll need to do this anyway to 
support VTd for 32-bit (but I don't know if that's a reasonable thing to 
do anyway).

What do you think?

    J