add led_fw_path to the permitted_list

[habeebmohammed]

Description
I tried to add led_fw_dir path to the config.bcm file and got an error when building.
"D7264Q28B/th2-d7264q28b-64x100g.config.bcm PASSED the config check!
[line 2] Error: led_fw_dir is not permitted"

adding LED binaries for TH3 & TD3.
https://groups.google.com/forum/#!topic/sonicproject/nlTkUQ53DbM

Steps to reproduce the issue:

1. add led_fw_path to config.bcm file
2. build the code
   3.Error: led_fw_path is not permitted

Describe the results you received:
[line 2] Error: led_fw_dir is not permitted

Describe the results you expected:
No error should be seen when building the code.

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

```
(paste your output here)
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**Attach debug file `sudo generate_dump`:**

```
(paste your output here)
```