sensor_scmi: Use sensor element name

Use directly sensor element names rather than through the sensor
info structure. Remove the name field in the sensor info structure
as it is not needed anymore.

Change-Id: I53a9a0ed2cca7a59c53ecde0dce27deb8da6c7b6
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>