thermal: ti-bandgap: Add support for temperature sensor

The dra7xx series of SOCs contain a temperature sensor and an
associated analog-to-digital converter (ADC) which produces
an output which is proportional to the SOC temperature.
Add support for this temperature sensor.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>