thermal: Default OF created trip points to writable

When registering a thermal zone from device tree, default the trip
points to writable. By default, only the root user can change these.

This allows the trip points to be tweaked after the system has
booted.

Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit 76af5495a5263a6d5b6d5de0f34d52de1dbf665e)
Signed-off-by: Kevin Hilman <khilman@linaro.org>