Here i'm facing the issue even after using the full UART (Tx, Rx, RTS, CTS).
These 4 pins are connected & configured from both (i.MX6UL & nrf52840_pca10056).
Note : nrf52840 chip is flashed with HCI_UART sample app of zephyr. In that i have configured 4 pins and make HWFC enable from nrf5.conf file.
Ans still i'm not facing this issue continuously. Lets say i'm keeping beacon scanning on for 4-5 hours then its working fine. It crashes when i keep it running for 2-3 days.