hwmon: (scpi) add energy meter support

SCPI specification v1.1 adds support for energy sensors. This patch
adds support for the same.

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