cmd: add temperature command

Currently, there is no way for users to check the readings from thermal
sensors from U-boot console, only some boards print it during boot.

So, lets add a simple "temperature" command that allows listing thermal
uclass devices and getting their value.

Note that the thermal devices are intenionally probed if list is used as
almost always they will not get probed otherwise and there is no way for
users to manually call probe on a certain device from console.

Assumption is made that temperature is returned in degrees C and not
milidegrees like in Linux as this is what most drivers seem to return.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Simon Glass <sjg@chromium.org>