clk: sunxi: Add "cpu" to list of protected clocks for sun5i

Now that the ahb clock on sun5i/sun7i is muxable, ahb is no longer
guaranteed to be a child of the cpu clock. Add the cpu clock to
the list of protected clocks so it doesn't get disabled.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>