Unlike MAX6646/MAX6647/MAX6649, MAX6648 and MAX6692 only support
a temperature range of 0..127 degrees C. Separate support for the
two sets of chips to be able to support maximum temperature ranges
correctly for all chips. Introduce new feature flag to indicate
temperature support up to 255 degrees C.

Since the chips are almost identical except for the supported temperature
range, automatic chip detection is limited. Effectively this means that
MAX6648 may be mis-detected as MAX6649 when auto-detected, but there is
nothing we can do about that.

Devicetree nodes are not added for the added chips since it is quite
unlikely that such old chips will ever be used in a devicetree based
system. They can be added later if needed.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>