rtc: ds1390: Add trickle charger device tree binding

Introduce a device tree binding for specifying the trickle charger
configuration for ds1390.

Signed-off-by: Ivan Grimaldi <grimaldi.ivan@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
diff --git a/drivers/rtc/Kconfig b/drivers/rtc/Kconfig
index 9d42906..57c2dbc 100644
--- a/drivers/rtc/Kconfig
+++ b/drivers/rtc/Kconfig
@@ -666,8 +666,8 @@
 	  If you say yes here you get support for the
 	  Dallas/Maxim DS1390/93/94 chips.
 
-	  This driver only supports the RTC feature, and not other chip
-	  features such as alarms and trickle charging.
+	  This driver supports the RTC feature and trickle charging but not
+	  other chip features such as alarms.
 
 	  This driver can also be built as a module. If so, the module
 	  will be called rtc-ds1390.