powerpc/setup_64: fix -Wempty-body warnings

At the beginning of setup_64.c, it has,

  #ifdef DEBUG
  #define DBG(fmt...) udbg_printf(fmt)
  #else
  #define DBG(fmt...)
  #endif

where DBG() could be compiled away, and generate warnings,

  arch/powerpc/kernel/setup_64.c: In function 'initialize_cache_info':
  arch/powerpc/kernel/setup_64.c:579:49: warning: suggest braces around
  empty body in an 'if' statement [-Wempty-body]
      DBG("Argh, can't find dcache properties !\n");
                                                 ^
  arch/powerpc/kernel/setup_64.c:582:49: warning: suggest braces around
  empty body in an 'if' statement [-Wempty-body]
      DBG("Argh, can't find icache properties !\n");

Fix it by using the suggestions from Michael:

  "Neither of those sites should use DBG(), that's not really early
  boot code, they should just use pr_warn().

  And the other uses of DBG() in initialize_cache_info() should just
  be removed.

  In smp_release_cpus() the entry/exit DBG's should just be removed,
  and the spinning_secondaries line should just be pr_debug().

  That would just leave the two calls in early_setup(). If we taught
  udbg_printf() to return early when udbg_putc is NULL, then we could
  just call udbg_printf() unconditionally and get rid of the DBG macro
  entirely."

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Qian Cai <cai@lca.pw>
[mpe: Split udbg change out into previous patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1563215552-8166-1-git-send-email-cai@lca.pw