of: document refcount incrementation of of_get_cpu_node()

This function increments refcount.  This is worth noting.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Frank Rowand <frank.rowand@am.sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>