Add Japanese specific plan for land mobile station.

[tooruuetani]

Summary

Add Japanese specific plan for land mobile station. Using from 920.6MHz to 923.4MHz with LBT.

A land mobile station can transmit at MAX EIRP 27 dBm. (Note) A user who installs land mobile station in Japan must apply to Japanese Ministry of Internal Affairs and Communications.

References #7, See https://github.com/TheThingsNetwork/lorawan-frequency-plans/pull/36 .

Changes

Currently AS_920_923 plan defines MAX EIRP 16 dBm, but some users want to use MAX EIRP 27 dBm for land mobile station. It is compliant to Japanese law ARIB-STD-T108, See attached table.

https://www.arib.or.jp/kikaku/kikaku_tushin/std-t108.html

Notes for Reviewers

Second plan "AS_920_923_JP_TTN_3_LAND_MOBILE" is for 16 channels gateway, so this plan is not intended to be used alone.

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Compatibility: The changes are backwards compatible, they don't break existing deployments.
• [x] Testing: The changes are tested.
• [x] Doc

[tooruuetani]

@htdvisser Thank you for comment.

'm fine with leaving the plan files as they are now, but since compared to https://github.com/TheThingsNetwork/lorawan-frequency-plans/pull/36 this only changes the max-eirp, why can't we extend the "base" plans, and only override the sub-bands (like we do for AS_920_923_LBT)?

It's just because #36 hasn't been merged yet. After merging #36 , I will modify this to extend the base plans.

[htdvisser]

OK. #36 was just merged. If you address the open comments and re-request review (or ping me) we'll try to also get this one into the v3.19 release of The Things Stack.

[tooruuetani]

OK, Thanks ! I will update this PR later.