overlays: i2c-rtc: Fix trickle-resistor-ohms param

The abx80x implementation of the trickle-resistor-ohms parameter is
missing the ":0" indicating that the target is an integer/cell value.

See: https://github.com/raspberrypi/linux/issues/3642



Signed-off-by: Phil Elwell <phil@raspberrypi.com>