clk: sunxi-ng: r40: Add minimal rate for video PLLs

According to documentation and experience with other similar SoCs, video
PLLs don't work stable if their output frequency is set below 192 MHz.

Because of that, set minimal rate to both R40 video PLLs to 192 MHz.

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>