thermal: tsens: Register virtual sensors

Virtual thermal zones will be used for GPU and CPU, where there are
more than one TSENS to monitor.

Register for virtual thermal zones once TSENS has registered individual
sensors.

Change-Id: Iec763408d97378f890e2f591bbf4a9419821fd69
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>