clk: sunxi-ng: h6: Allow GPU to change parent rate

GPU PLL was designed with dynamic frequency switching in mind so driver
can adjust rate based on the GPU load.

Allow GPU clock to change parent rate (GPU PLL is the only possible
parent of GPU clock).

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <mripard@kernel.org>