The clock driver has options that configure it's operation at build-time.

MAX_CLOCK_FREQ
This define, when set, identifies the running CPU clock rate of the system (change this to alter running CPU speed).
When this is not defined, The maximum clock rate for the CPU is used.

CRYSTAL_MAIN_FREQ_IN
This define is the external crystal frequency used for the main oscillator.

EXTCLKIN_FREQ_IN
This define is rate of the input clock signal on the CLKIN input of the device (on some parts only).