xtensa: fix format string warning in init_pmd

Use %lu instead of %zu to fix the following warning introduced with
recent memblock refactoring:
  xtensa/mm/mmu.c:36:9: warning: format '%zu' expects argument of type
  'size_t', but argument 3 has type 'long unsigned int

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>