drivers: thermal: of-thermal: Add support for getting trip temperature

Add support for the sensor drivers using of-thermal interface to support
reading the trip temperature from the hardware using a callback. This
can be used in case, when the hardware works on a pre-configured
threshold different from the threshold set by software.

Change-Id: Ic5aaf1586b8dcbb3da0dd775718407c257b2064f
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>