Update to SONiC "Porting Guide" document - platform_asic file required

[Qi Luo]

Hi SONiC community,

We recently update "Porting Guide" document (https://github.com/Azure/SONiC/wiki/Porting-Guide) with a new required file `platform_asic` in each device platform folder.

Please check the document difference https://github.com/Azure/SONiC/wiki/Porting-Guide/_compare/158f2105a98e080c97051f83b44e13e7db01b28c...c10ffad508da73612d20a3c9cbfa293e73be7321

There is new unit test case added. So if this file is missing in any new platform folder, build will fail. We already added that file to existing platform folders in master branch.

Best regards,

-Qi