Joachim,
Accessing the thermal register is covered in the Thermal Management white paper:

support.logicpd.com/downloads/1522/1021014B_WP491_DM3730-AM3703_Torpedo_SOM_Thermal_Management.pdf

From Linux:

3.1.1.2 Linux and Android
To read the temperature register in Linux, use the command below. NOTE: All Linux commands in this document are based on Logic PD’s DM37x Linux BSP version 2.2-2.
DM-37x# cat /sys/devices/platform/temp_sensor/celsius
18.0

We do not have instructions for u-boot, but I believe you can use the Windows CE instructions to write/read to the internal registers from the u-boot command line.

3.1.1.3 Windows CE
To set up the temperature register and read it back in Windows CE, use the commands below. NOTE: All Windows CE commands in this document are based on Logic PD’s DM37x Windows Embedded CE 6.0 BSP version 3.0.0.
\> cd \WINDOWS
\> do out32 48004A08 2
\> do out32 48002524 600
\> do in32 48002524

The TI TRM has information on interpreting the results.

Thanks for your answer. Before posting I've taken a look to 'Articles -- Technical FAQs about our TI products' folder and it was no document about temperature however I haven't taken a look to the download area.

Thanks and Best Regards,
Joaquim Duran