drivers: cc13xx_cc26xx: pwm: Add polarity flag support

- Allow setting polarity from DT.
- At least on bcf, going from 0 to 255 causes brightness to decrease
  (although 0 is still off and 255 is still on).
- Seems to work as expcted by using TnPWML register.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>