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>