arm64: dts: rockchip: Drop RTC clock-frequency on rk3588-rock-5b

The hym8563 RTC driver doesn't handle the 'clock-frequency' property,
which is also indicated by the following dtbs_check warning:

  rk3588-rock-5b.dtb: rtc@51: Unevaluated properties are not allowed ('clock-frequency' was unexpected)
  From schema: Documentation/devicetree/bindings/rtc/haoyu,hym8563.yaml

Drop the unsupported property.

Fixes: 1e9c2404 ("arm64: dts: rockchip: Enable RTC support for Rock 5B")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230414125425.124994-2-cristian.ciocaltea@collabora.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>