#### Why I did it
Add platform_asic file to each platform folder in sonic-device-data package. The file content will be used as the ground truth of mapping from PLATFORM_STRING to switch ASIC family.
One use case of the mapping is to prevent installing a wrong image, which targets for other ASIC platforms. For example, currently we have several ONIE images naming as sonic-*.bin, it's easy to mistakenly install the wrong image. With this mapping built into image, we could fetch the ONIE platform string, and figure out which ASIC it is using, and check we are installing the correct image.
After this PR merged, each platform vendor has to add one mandatory text file `device/PLATFORM_VENDOR/PLATFORM_STRING/platform_asic`, with the content of the platform's switch ASIC family.
I will update https://github.com/Azure/SONiC/wiki/Porting-Guide after this PR is merged.
You can get a list of the ASIC platforms by `ls -b platform | cat`. Currently the options are
```
barefoot
broadcom
cavium
centec
centec-arm64
generic
innovium
marvell
marvell-arm64
marvell-armhf
mellanox
nephos
p4
vs
```
Also support
```
broadcom-dnx
```
#### How I did it
#### How to verify it
Test one image on DUT. And check the folders under `/usr/share/sonic/device`
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