arm64: dts: imx8mp-debix-model-a: Remove invalid rtc property

Per haoyu,hym8563.yaml, 'clock-frequency' is not a valid property.

Remove it to fix the following schema warning:

imx8mp-debix-model-a.dtb: rtc@51: Unevaluated properties are not allowed ('clock-frequency' was unexpected)

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>