arm: enable data-cache at the same time as enabling the MMU, not before

arm: enable data-cache at the same time as enabling the MMU, not before

With enough warnings enabled the model seemed to be complaining that pages
cached before paging was enabled had been mapped with to inconsistent sets of
attributes. I'm not convinced that isn't a model issue, nor am I convinced
this has really fixed anything, but it seems sensible enough.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>