Some chips use different exponents for sensors on different pages
or rails. Detect and store exponent per page to support this situation.

This fixes a problem with wrong voltages seen on UCD90120.
Reported-by: NSoren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: NSoren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>