hwmon: (pmbus) Support per-page exponent in linear mode

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: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>