clk: sunxi-ng: Add check for minimal rate to NM PLLs

Some NM PLLs doesn't work well when their output clock rate is set below
certain rate.

Add support for that constrain.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>