thermal: core: Add Kconfig option to enable writable trips

Add a Kconfig option to allow system integrators to control whether
userspace tools can change trip temperatures. This option overrides
the thermal zone setup in the driver code and must be enabled for
platform specified writable trips to come into effect.

The original behaviour of requiring root privileges to change trip
temperatures remains unchanged.

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>