thermal: expose thermal_zone_get_temp API

This patch exports the thermal_zone_get_temp API so that driver
writers can fetch temperature of thermal zones managed by other
drivers.

Acked-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>