hwmon: Support sensors exported via ARM SCP interface

Create a driver to add support for SoC sensors exported by the System
Control Processor (SCP) via the System Control and Power Interface
(SCPI). The supported sensor types is one of voltage, temperature,
current, and power.

The sensor labels and values provided by the SCP are exported via the
hwmon sysfs interface.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Cc: Sudeep Holla <sudeep.holla@arm.com>