twl-regulator: Add turnon delay to reg_enable

This change implements a basic turnon delay in the regulator enable function
to make it less probable that reg_enable returns before the regulator
output is at target level

Signed-off-by: Juha Keski-Saari <ext-juha.1.keski-saari@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>