thermal: thermal_sys: Add support for configurable trip points.

Add functionality for configurable hi, low and critical low.
Change the trip point attributes to allow userspace clients
with root access to set temperature for configurable hi and
low temperature.

Change-Id: I25c9c3bcfd58e44da5369187d1095559062f1860
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>