clk: sunxi-ng: r40: Allow setting parent rate to display related clocks

Display related peripherals need precise clocks to operate correctly.

Allow DE2, TCONs and HDMI to set parent clock.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>