regulator: core: Support bypass mode

Many regulators support a bypass mode where they simply switch their
input supply to the output. This is mainly used in low power retention
states where power consumption is extremely low so higher voltage or
less clean supplies can be used.

Support this by providing ops for the drivers and a consumer API which
allows the device to be put into bypass mode if all consumers enable it
and the machine enables permission for this.

This is not supported as a mode since the existing modes are rarely used
due to fuzzy definition and mostly redundant with modern hardware which is
able to respond promptly to load changes.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reviewed-by: Graeme Gregory <gg@slimlogic.co.uk>