Make default xmit power for Japan 13 dBm

[geeksville]

Based on comment by Yuki:

According to Japanese rules.

Max 1mW 915MHz - 930MHz No radio license registration
Max 20mW 920.6MHz - 928MHz No radio license registration
Max 250mW 920.6MHz - 923.4MHz Radio license registration required

It would be great if you could change the default powerlevel and
frequency band from the Android app.

and https://www.everythingrf.com/rf-calculators/dbm-to-watts

It seems like we should set the default xmit power when in Japan to 13 dBm. Probably by adding a REGION_DEFAULT_POWER definition?

[geeksville]

This issue has been mentioned on Meshtastic. There might be relevant details there:

https://meshtastic.discourse.group/t/about-the-japanese-version/1171/6

[geeksville]

This issue has been mentioned on Meshtastic. There might be relevant details there:

https://meshtastic.discourse.group/t/about-the-japanese-version/1171/11