Added LoRa Thing Plus variant

[dandobs]

Added target for LoRa thing plus in mbed to support Arduino core.
Submitting along with PR sparkfun/Arduino_Apollo3#305, adding the LoRa thing plus variant to Arduino core

[oclyke]

Hi @danJTDC,
Thanks for making this PR. I can see that there are a lot of unrelated changes included. I would recommend reducing this PR to only the changes that were required to add the variant. This could involve several strategies including:

• rebasing off the latest 'ambiq-apollo3-arduino' branch
• starting fresh from the 'ambiq-apollo3-arduino' branch and applying only the necessary changes
• or any number of other methods - lots of ways to skin a cat!

@Wenn0101 do you have any other comments on this?

Getting this PR pulled in is a prerequisite for Arduino Apollo3 PR #305

[dandobs]

Hi @oclyke,
I updated my fork to the latest version and made only the changes required to add the variant.
This was closed automatically in the process, I submitted a new PR with those changes #45