clk: sunxi-ng: sun5i: Add clk_set_rate_parent to the CPU clock

The current CPU clock is missing the option to change the rate of its
parents, leading to improper rates calculated by cpufreq, and eventually
crashes.

Cc: <stable@vger.kernel.org>
Fixes: 5e737617 ("clk: sunxi-ng: Add sun5i CCU driver")
Reported-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>