thermal: armada: average over samples to avoid glitches

Configure the sample frequency and number of averaged samples.

This is needed for two reasons:
1/ To be bootloader independent.
2/ To prepare the introduction of multi-sensors support by preventing
   inconsistencies when reading temperatures that could be a mean of
   samples took from different sensors.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>