This commit:

* Moves all cpu files from cpu/cc26xx to cpu/cc26xx-cc13xx
* Bumps the CC26xxware submodule to the latest TI release
* Adds CC13xxware as a submodule
* Adds support for sub-ghz mode / IEEE 802.15.4g
* Splits the driver into multiple files for clarity. We now have the following structure:
  * A common module that handles access to the RF core, interrupts etc
  * A module that takes care of BLE functionality
  * A netstack radio driver for IEEE mode (2.4GHz)
  * A netstack radio driver for PROP mode (sub-ghz - multiple bands)

This commit also adds tick suppression functionality, applicable to all chips of the CC26xx and CC13xx families. Instead waking up on every clock tick simply to increment our software counter, we now only wake up just in time to service the next scheduled etimer. ContikiMAC-triggered wakeups are unaffected.

Laslty, this commit also applies a number of minor changes:
* Addition of missing includes
* Removal of stub functions
* Removal of a woraround for a CC26xxware bug that has now been fixed