thermal: qcom-spmi: Treat reg property as a single cell

We only read the first element of the reg property to figure out
the offset of the temperature sensor inside the PMIC.
Furthermore, we want to remove the second element in DT, so just
don't read the second element so that probe keeps working if we
change the DT in the future.

Cc: Ivan T. Ivanov <iivanov.xz@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>