hwmon: (pmbus/max20730) adjust the vout reading given voltage divider

Problem:
We use voltage dividers so that the voltage presented at the voltage
sense pins is confusing. We might need to convert these readings to more
meaningful readings given the voltage divider.

Solution:
Read the voltage divider resistance from dts and convert the voltage
reading to a more meaningful reading.

Testing:
max20730 with voltage divider

Signed-off-by: Chu Lin <linchuyuan@google.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20201004031445.2321090-3-linchuyuan@google.com


[groeck: Return -EINVAL instead of -ENODEV on bad deevicetree data]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>