drivers: thermal: Add attributes to show the cdev limits

At the moment, there is no method for the userspace to know the operating
upper and lower limit for a cooling device associated with a trip.

Add new sysfs attributes cdev#_upper_limit and cdev#_lower_limit to
display the operating upper and lower limit of the cooling device.

Change-Id: Ifda1c51f8ebaf071552d454a4f1b6d07f4e9d005
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>