rtc-ab8500-add-calibration-attribute-to-ab8500-rtc-v3

rtc-ab8500-add-calibration-attribute-to-ab8500-rtc-v3

ChangeLog v2->v3:
- back to square 1. 0x80 is not allowed because the representation
  is not two's complement but bit 7 is a sign bit, thus 0x80 is
  just another way to say "zero". Sorry for the mess, clarified this
  with a comment in the code.

Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Mark Godfrey <mark.godfrey@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>