Set denominator to 1 if it is 0 in the init phase.

This prevents panic and also helps when the clocks
become available later.
This is based on comment from manu@ on
https://reviews.freebsd.org/D31299

sys/arm64/rockchip/clk/rk_clk_fract.c

@@ -150,6 +150,8 @@ rk_clk_fract_init(struct clknode *clk, device_t dev)
 
 	sc->numerator  = (reg >> 16) & 0xFFFF;
 	sc->denominator  = reg & 0xFFFF;
+	if (sc->denominator == 0)
+		sc->denominator = 1;
 	clknode_init_parent_idx(clk, 0);
 
 	return(0);