[C26] let external RTC be system default

Assign SoC RTC to rtc1, so module RTC PCF2123 can be assigned
to system default rtc0.

arch/arm64/boot/dts/seco/seco-imx8qm-c26.dts

@@ -12,6 +12,9 @@
 	model = "SECO i.MX8QM C26";
 	compatible = "seco,imx8qm-c26", "fsl,imx8qm";
 
+	aliases {
+		rtc1 = &rtc;
+	};
 
 	chosen {
 		bootargs = "console=ttyLP0,115200 earlycon=lpuart32,0x5a060000,115200";