ARM: dts: aspeed: ast2500evb: Enable built in RTC

Enable this device so the RTC driver can be tested on the EVB.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20210727034639.474458-1-joel@jms.id.au

arch/arm/boot/dts/aspeed-ast2500-evb.dts

@@ -129,3 +129,7 @@
      status = "okay";
      memory-region = <&gfx_memory>;
 };
+
+&rtc {
+	status = "okay";
+};