thermal: allow registering without .get_temp

This patch changes the thermal core driver to allow
registration of thermal zones without the .get_temp callback.

The idea behind this change is to allow lazy registration
of sensor callbacks.

The thermal zone will be disabled whenever the ops
does not contain a .get_temp callback. The sysfs interface
will be returning -EINVAL on any temperature read operation.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>