xtensa: remove mem_domain excess padding

The ptables field in arch_mem_domain for Xtensa has excessive
padding as it is incorrectly marked with needing page size
alignment. This is simply a pointer and not the actual page
table so there is no need for that alignment. So remove it.

Fixes #71896

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
(cherry picked from commit eb9fc642)