drivers: thermal: allow ti-soc-thermal run without pcb zone

This patch changes the behavior of TI SoC thermal driver
when there is a PCB thermal zone.

Instead of reporting an error code when reading from
PCB temperature sensor fails, this patch will make
the driver attempt to compose the hotspot extrapolation
based on bandgap readings only.

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>