regulator: ab8500: Add new operations for Vaux3

In former functions, only can set Vaux3 to 2.91V, because the
highest bit of Vaux3 register is put into another register. So
add new expanded functions for Vaux3's operation.

Signed-off-by: zhang xiaomei <xiaomei.zhang@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Philippe LANGLAIS <philippe.langlais@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>