Add method Si5351::reset_freq()

[KT315AM]

This PR adds the new method Si5351::reset_freq() to the simplify logic of the frequency manipulation.
Current API covers only cases when CLK has fixed output frequency.
Introduced method reset_freq() allows to cover cases when CLK has variable output frequency. It's eliminate needs to manual call output_enable() method to enable CLK before set new frequency value.

Initially was idea to name method reset_clock(), but CLK it's not only output frequency - it also has other settings.
So method was named reset_freq() meaning that resets only output frequency.

Example

Current API:

Si5351 si5351;
si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
...
si5351.set_freq(freq1, clk);
si5351.set_freq(freq2, clk);
si5351.output_enable(clk, 0);
...
si5351.output_enable(clk, 1); // <- redundancy code
si5351.set_freq(freq3, clk);
si5351.output_enable(clk, 0);
...
si5351.output_enable(clk, 1); // <- redundancy code
si5351.set_freq(freq4, clk);

API with new method reset_freq():

Si5351 si5351;
si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
...
si5351.set_freq(freq1, clk);
si5351.set_freq(freq2, clk);
si5351.reset_freq(clk);
...
si5351.set_freq(freq3, clk);
si5351.reset_freq(clk);
...
si5351.set_freq(freq4, clk);