ieee802154: cc13xx_cc26xx: sub_ghz support

[cfriedt]

Is your enhancement proposal related to a problem? Please describe.
The ieee802154 cc13xx_cc26xx could probably also use an option for Sub GHz support as well. This might be a little easier to do once #26312 is done as well, since the rf api is (somewhat?) common.

It would be helpful to also have a choice in Kconfig or DT for the radio frequency.

Edit: According to the TRM section 25.10.5.2.1, IEEE 802.15.4g (sub ghz) mode is supported via the "proprietary" radio. It does not seem that the IEEE mode radio commands are used at all and that the PROP commands are used for setup, TX, RX, and CS.

Describe the solution you'd like
After #26312 is done, I could probably take a crack at this.

Describe alternatives you've considered

Additional context

Depends on #26312

[cfriedt]

This issue has a PR ready to be made, but it is blocked by #26312.

The same issue blocks https://github.com/cfriedt/greybus-for-zephyr and https://github.com/jadonk/beagleconnect