xen/arm: Support dtb /memreserve/ regions

xen/arm: Support dtb /memreserve/ regions

This requires a mapping of the DTB during setup_mm. Previously this was in
the BOOT_MISC slot, which is clobbered by setup_pagetables. Split it out
into its own slot which can be preserved.

Also handle these regions as part of consider_modules() and when adding pages
to the heaps to ensure we do not locate any part of Xen or the heaps over
them.

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