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>

arch/arm64/boot/dts/freescale/imx8mp-debix-model-a.dts

@@ -226,7 +226,6 @@ rtc@51 {
 		compatible = "haoyu,hym8563";
 		reg = <0x51>;
 		#clock-cells = <0>;
-		clock-frequency = <32768>;
 		clock-output-names = "xin32k";
 		interrupt-parent = <&gpio2>;
 		interrupts = <11 IRQ_TYPE_EDGE_FALLING>;