MIPS: DTS: jz4780: add #clock-cells to rtc_dev

This makes the driver present the clk32k signal if requested.
It is needed to clock the PMU of the BCM4330 WiFi and Bluetooth
module of the CI20 board.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>