Hello Amit:
I got confused with your statement:
After all effort the maximum speed I could get is around 25 Mhz in place of 24 MHz.
Do you mean you need more than 25 MHz? or that the maximum you are getting is 24 MHz?
If you look at the K20 50 MHz datasheet, you can see the maximum frequency for SPI module is 25 MHz:

Hope this is somewhat useful.
Regards!
/Jorge Gonzalez