scmi sensor: Sign Change for Sensor Value

This patch will change the sensor value returned from two
unsigned 32 bit values, to two signed 32 bit values.

This is being done to be compliant with SCMI V3, which now
supports a signed sensor value.

Signed-off-by: Katherine Vincent <katherine.vincent@arm.com>
Change-Id: I6191efb31023f358a2eafad97c64e80c957207e5