xen: arm: rename p2m->first_level to p2m->root.

xen: arm: rename p2m->first_level to p2m->root.

This was previously part of Vijaya's "xen/arm: Add 4-level page table
for stage 2 translation" but is split out here to make that patch
easier to read.

I went with ->root rather than ->root_level as the original did.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Cc: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>