regulator: core: Provide standard enable operations for regmap users

Since the enable(), disable() and is_enabled() operations for most regmap
based regulators come down to reading and updating a single register bit
we can factor out the code and allow these drivers to just define which
bit to update using the enable_reg and enable_mask fields in their desc
and then use operations provided by the core.

As well as the code saving this opens the door to future optimisation of
the bulk operations - if the core can realise that we are updating a
single register for multiple regulators then it should be able to combine
these updates into a single physical operation.

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