board/BuR/common: add br resetcontoller implementation

On many B&R boards we have a reset-controller, responsible for very
early board-bringup (voltages, clocks, ...) and bootmode selection.

To be ready for adding more B&R boards to source tree while avoiding
duplicate code, we add the resetcontroller implementation to the common
part of B&R boards.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>