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>

arch/xtensa/mm/mmu.c

@@ -33,7 +33,7 @@ static void * __init init_pmd(unsigned long vaddr, unsigned long n_pages)
 
 	pte = memblock_alloc_low(n_pages * sizeof(pte_t), PAGE_SIZE);
 	if (!pte)
-		panic("%s: Failed to allocate %zu bytes align=%lx\n",
+		panic("%s: Failed to allocate %lu bytes align=%lx\n",
 		      __func__, n_pages * sizeof(pte_t), PAGE_SIZE);
 
 	for (i = 0; i < n_pages; ++i)