U-Boot has code to replace the infamous Freescale bootlet code.
This patch adds this for barebox with some changes:
- Separate it more into mx23/mx28 functions instead of mxs functions
  with #ifdefs for the actual SoC
- Add mx2x_power_init_battery_input() power entry point for boards
  which have a regulated input on the battery pin to supply the board.
- Export more functions to be more flexible when boards need non standard
  setup.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>