regulator: Enable supply regulator if child rail is enabled.

During regulator_register, the rail is set on the provided
machine constraints and if it is enabled then it is also
require to enable the supply regulator. This will make sure
that:
   1. Proper reference count for supply regulator to be maintain.
   2. Supply regulator should be enable when given rail is enabled.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>