drivers: adc: enhance millivolt conversion with 64-bit support

This commit updates the adc_raw_to_millivolts function to use 64-bit
arithmetic. It is necessary when the intermediate result is higher
than the maximum value of uint32_t. It also adds a range check to
ensure the conversion result does not exceed the limits of int32_t,
providing an assertion message for out-of-range values.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>