ARM: dts: at91: sama5d2: add an rtc label for derived boards

Add an rtc label so we just need to alias it from derived boards.
Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Link: https://lore.kernel.org/r/20191002145914.14874-1-kamel.bouhara@bootlin.comSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

arch/arm/boot/dts/sama5d2.dtsi

@@ -689,7 +689,7 @@ clk32k: sckc@f8048050 {
 				#clock-cells = <0>;
 			};
 
-			rtc@f80480b0 {
+			rtc: rtc@f80480b0 {
 				compatible = "atmel,at91rm9200-rtc";
 				reg = <0xf80480b0 0x30>;
 				interrupts = <74 IRQ_TYPE_LEVEL_HIGH 7>;