regulator: ab8500: Remove the need for a 'delay' property

The regulator framework now has provisions to read a regulator's
start-up time via the 'enable_time' attribute. So let's remove all
'enable_time'/'delay' duplication and just stick with 'enable_time'
from now on.

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