regulator: ab8500: Set enable enable_time in regulator_desc

Allow the regulator framework to directly access the time it takes
any given regulator to settle. This saves time and code because
without it we would have to implement a dedication function to read
the value from elsewhere.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>