xen/arm: p2m: Correctly initialize cur_offset

xen/arm: p2m: Correctly initialize cur_offset

{~0,} only initializes the first cell of the array to ~0. The other cells
are initialized to 0.

Explicitly initialize every cells of the array and, at the same time, do the
same for the mappings.

This is fixing boot after 82985d7 "xen: arm: handle variable p2m levels
in apply_p2m_changes" on platform where the root-level doesn't have
concatenate table (such as the Foundation Model).

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>