arm64/mm: fix kernel-doc comments

Building a kernel with W=1 generates several warnings due to abuse of
kernel-doc comments:

  | arch/arm64/mm/numa.c:281: warning: Cannot understand  *
  |   on line 281 - I thought it was a doc line

Tidy up the comments to remove the warnings.

Fixes: 1a2db300 ("arm64, numa: Add NUMA support for arm64 platforms.")
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Will Deacon <will.deacon@arm.com>