For DP83TC812S/R and DP83TC814-Q1 if clkout pin is not being used to provide a clock signal to another component, then it is recommended to shut down this clock output and reduce its impact on EM emissions.
Clkout's output can be made static (no-toggling) by programming following registers :
Reg<0x045F> = 0x000F
Reg<0x0453> = 0x0003
For DP83TC813-Q1, these are the register writes:
Reg<0x452> = 0x0303
Reg<0x045F> = 0x000F
Reg<0x0453> = 0x0003