clk: sunxi-ng: Use u64 for calculation of NM rate

Allwinner H6 SoC has multiplier N range between 1 and 254. Since parent
rate is 24MHz, intermediate result when calculating final rate easily
overflows 32 bit variable.

Because of that, introduce function for calculating clock rate which
uses 64 bit variable for intermediate result.

Fixes: 6174a1e2 ("clk: sunxi-ng: Add N-M-factor clock support")
Fixes: ee28648c ("clk: sunxi-ng: Remove the use of rational computations")

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