Add Tanmatsu variant

[renzenicolai]

This pull request adds support for Tanmatsu by adding a variant for the board.

Early revisions of the Tanmatsu hardware use XTAL clock source while newer revisions use TCXO, hence the need for TCXO_OPTIONAL. There are two board variants, one with SX1262 based LoRa module for 868/915 MHz bands and one with SX1268 based LoRa module for 433MHz band.

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions (it only adds a variant)

[CLAassistant]

All committers have signed the CLA.