arm: dts: rockchip: remove clock-frequency from rtc

[ Upstream commit 6122f3be

 ]

'clock-frequency' is not part of the DT binding and not supported by the
Linux driver.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20221024165549.74574-5-sebastian.reichel@collabora.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>