arm64: dts: imx8mm-var-som-symphony: Drop wake-up source from RTC

The RTC on Symphony board does not have its interrupt pin connected to
the SoC, therefore it is not capable of waking up.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mm-var-som-symphony.dts

@@ -140,7 +140,6 @@
 	rtc@68 {
 		compatible = "dallas,ds1337";
 		reg = <0x68>;
-		wakeup-source;
 	};
 };