hwmon: (adt7475) document mapping of sysfs entries to inputs

As per the usual standard with hwmon drivers the mapping to sysfs
entries follows the register map of the device e.g. in0_input
corresponds to the register 0x20, in1_input corresponds to 0x21 etc.

Hardware designers tend to work with input pins instead of registers
which is where things start to get confusing. A hardware designer might
say "the 1.5V rail is connected to the VCCP pin" leaving the software
designer none the wiser as to which of the sysfs entries should be
associated with the label "1.5V".

Try to bridge the gap by documenting the mapping of sysfs entries to
the corresponding pins. This should allow someone to create a
configuration file or other mapping without needing to dive into the
code and ADT datasheets.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>