regulator: Implement deferred disable support

It is a reasonably common pattern for hardware to require some delay after
being quiesced before the disable has finalised, especially in mixed signal
devices. For example, an active discharge may be required to ensure that
the circuit starts up again in a known state. Avoid having to implement
such delays in the regulator API by providing regulator_deferred_disable()
which will do a regulator_disable() a specified number of milliseconds
after it is called.

Due to the reference counting done on regulators a deferred disable can
be cancelled by doing another regulator_enable().

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>