Hi Prasanna Naik ,
MK22FN512VLL12 can download the according SDK, and in the middleware, select the KBOOT, then you will find the flash bootloader code.

More details about the KBOOT, you can refer to the document in this link:
MCU Bootloader | NXP
About the LPC LPC54608J512BD208 , actually, this chip already have the on chip ISP function for UART, you can refer to the chip user manual:
https://www.nxp.com/webapp/Download?colCode=UM10912
Chapter 5: LPC546xx ISP and IAP
Wish it helps you!
Kerry
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