Refactor : Organize portable_common directory file descriptions

Organize `portable_common` directory file descriptions into a table format
Signed-off-by: Adarsh <Adarshbunny293@gmail.com>

backends/bmv2/portable_common/README.md

@@ -2,20 +2,11 @@
 
 This directory contains reusable components common to both the `psa_switch` and `pna_nic` backends.
 
-### midend.h, midend.cpp
 
-Defines the common mid-end processing of both the `psa_switch` and `pna_nic` backends.
 
-### options.h, options.cpp
-
-Defines the common command-line options of both the `psa_switch` and `pna_nic` backends.
-
-### portable.h, portable.cpp
-
-Defines common functionalities that generate representations of P4 programs.
-
-The files `portableProgramStructure.h` and `portableProgramStructure.cpp` are in the `backends/common` directory.
-
-### portableProgramStructure.h, portableProgramStructure.cpp
-
-Defines and implements the common program structure of both the `psa_switch` and `pna_nic` backends.
+|      **File Name**          |      **Description**     |
+|-----------------------------|--------------------------|
+| **midend.h, midend.cpp**    | Defines the common mid-end processing of both the `psa_switch` and `pna_nic` backends.    |
+| **options.h, options.cpp**  | Defines the common command-line options of both the `psa_switch` and `pna_nic` backends.   |
+| **portable.h, portable.cpp**| Defines common functionalities that generate representations of P4 programs.   |
+| **portableProgramStructure.h, portableProgramStructure.cpp** | Defines and implements the common program structure of both the `psa_switch` and `pna_nic` backends. _These files are located in the `backends/common` directory._ |