firmware: arm_scmi: fetch and store sensor scale

In preparation for dealing with scales within the SCMI HWMON driver,
fetch and store the sensor unit scale into the scmi_sensor_info
structure. In order to simplify computations for upper layer, take care
of sign extending the scale to a full 8-bit signed value.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
[sudeep.holla: update bitfield values as per specification]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>