Added post commands for enabling fifos

[nissampa]

What I did

     - Updated dpu-tty.py to run fifo enable configuration commands after exiting from dpu console.

How I did it

     - Read the command lists from platform.json if present and execute it.
     - Command to Write to fifos_enable in Sysfs path

How to verify it

     - Added the steps in UT

Previous command output (if the output of a command-line utility has changed)

     - No change

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[nissampa]

UT_dpu_tty_fifos.txt