clk: sunxi-ng: nkmp: Add constraint for maximum rate

Some, if not most, NKMP PLLs can be set to higher rate that is really
supported by HW.

Implement support for maximum frequency constrain for NKMP PLLs.

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