thermal/drivers/cpuidle_cooling: Use device name instead of auto-numbering

Currently the naming of a cooling device is just a cooling technique
followed by a number. When there are multiple cooling devices using
the same technique, it is impossible to clearly identify the related
device as this one is just a number.

For instance:

 thermal-idle-0
 thermal-idle-1
 thermal-idle-2
 thermal-idle-3
 etc ...

The 'thermal' prefix is redundant with the subsystem namespace. This
patch removes the 'thermal prefix and changes the number by the device
name. So the naming above becomes:

 idle-cpu0
 idle-cpu1
 idle-cpu2
 idle-cpu3
 etc ...

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20210314111333.16551-4-daniel.lezcano@linaro.org