regulator: helpers: Modify helpers enabling multi-bit control

This patch extends the regulator helpers to account for device that use
multiple bits for control when using regmap enable/disable/bypass ops.

The actual regulator helpers wrongly assume that the regulator control
is always performed using single bits, using in the regulator_desc
struct only two parameters *_reg and *_mask defining register and mask
for control.

This patch extends this struct and introduces the helpers to take into
account devices where control is performed using multiple bits and
specific multi-bit values are used for enabling/disabling/bypassing the
regulator.

Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Mark Brown <broonie@linaro.org>