ab8500: Add devicetree support for fuelgauge

- This patch adds device tree support for fuelgauge driver
- optimize bm devices platform_data usage and of_probe(...)
  Note: of_probe() routine for battery managed devices is made
  common across all bm drivers.
- test status:
  - interrupt numbers assigned differs between legacy and FDT mode.

Signed-off-by: Rajanikanth H.V <rajanikanth.hv@stericsson.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>