ab8500-charger: Add backup battery charge voltages on the ab8540

Add 2.7v, 2.9v, 3.0v, 3.2v and 3.3v charging voltages for backup
battery. Before that only 2.5v, 2.6v, 2.8v, 3.1v were available.

Signed-off-by: Yang QU <yang.qu@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Maxime COQUELIN <maxime.coquelin@stericsson.com>
Reviewed-by: Marcus COOPER <marcus.xm.cooper@stericsson.com>
Tested-by: Xiao Mei ZHANG <xiaomei.zhang@stericsson.com>