Add support for allocating private address space instances. Targets that
support per-context pagetables should implement their own function to
allocate private address spaces.

The default will return a pointer to the global address space.
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>