overlays: Add trickle-voltage-mv parameter to RTCs

The RV3032 RTC requires an additional DT property to enable trickle
charging. Add a parameter - trickle-voltage-mv - to the i2c-rtc
and i2c-rtc-gpio overlays to set it.

See: https://github.com/raspberrypi/linux/issues/5547



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