mm: add mm_largest api to get the current largest available memory block

Signed-off-by: yinshengkai <yinshengkai@xiaomi.com>

mm/tlsf/mm_tlsf.c

@@ -1476,3 +1476,16 @@ size_t mm_heapfree(FAR struct mm_heap_s *heap)
 {
   return heap->mm_heapsize - heap->mm_curused;
 }
+
+/****************************************************************************
+ * Name: mm_heapfree_largest
+ *
+ * Description:
+ *   Return the largest chunk of contiguous memory in the heap
+ *
+ ****************************************************************************/
+
+size_t mm_heapfree_largest(FAR struct mm_heap_s *heap)
+{
+  return SIZE_MAX;
+}