xen/arm: move setup_mm right after setup_pagetables

xen/arm: move setup_mm right after setup_pagetables

At the moment we destroy the DTB mappings we have in setup_pagetables
and we restore them only in setup_mm.

Move setup_mm right after setup_pagetables.
This ensures we have a valid DTB mapping while running the subsequent
initialization code.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- s/atag_paddr/fdt_paddr/ ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>