Enables operation of voltage converter.
Remark 
1.      About 1 microsecond of stabilization time is 
required when changing the voltage converter from operation stopped to operation 
enabled status.
Consequently, about 1 micro second must be left free between 
the call to this API function and the call to R_ADC_Start.
Remark 
2.      On the [A/D Converter], in the [Comparator 
operation setting] area, if "Operation" is selected, then the voltage converter 
will be switched to "always on".
There is thus no need to call this API 
function in this case.
[Syntax]
void R_ADC_Set_OperationOn ( void );
[Argument(s)]
None.
[Return value]
None.