thermal: armada: fix formula documentation comment

The formula implementation at armada_get_temp() indicates that the sign
in the formula is inverted.

Cc: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>