module: Voltage Domain driver and related SCMIv3 protocol driver

Introduces a basic Voltage Domain driver and a SCMI voltage domain
protocol driver. SCMI voltage domain protocol is introduced in SCMI
specification v3.

Module voltage_domain is a generic HAL for voltage domains, aka power
supply regulators drivers. Based on module configuration, the module
binds each element to the voltage domain backend driver thru a generic
driver interface:
- set & get config, using SCMI voltage domain configuration format.
- set & get voltage level, in microvolt (uV)

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>