arm: map device tree blob in initial page tables

arm: map device tree blob in initial page tables

Add a mapping for the device tree blob in the initial page tables.
This will allow the DTB to be parsed for memory information prior to
setting up the real page tables.

It is mapped into the first L2 slot after the fixmap.  When this slot
is reused in setup_pagetables(), flush the TLB.

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