rtc: ds1307: simplify hwmon config

We don't have to define an extra config symbol, IS_REACHABLE does
what we need. And having this config symbol just to save the few
bytes of hwmon support on non-DS3231 chips isn't worth it IMO
(especially as the symbol is set per default).

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>