I would start with reading the ADC section of the user manual.
The sampling frequency is determined by your method of triggering a conversion.
I am using a timer in one of my applications (for the LPC54628):
adcConvSeqConfigStruct.triggerMask = ADC_TRG_CTIMER0_MAT3;
Enabling/running the timer will start the sampling.
The ADC clock determines only the ADC-internal timing, e.g. sampling and conversion times.