test: bootcount: add bootcount-uclass test

Add a test for the bootcount uclass, which uses the RTC bootcount backend
(i.e. drivers/bootcount/rtc.c is implictly also tested).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/test.dts

@@ -315,6 +315,12 @@
 		};
 	};
 
+	bootcount@0 {
+		compatible = "u-boot,bootcount-rtc";
+		rtc = <&rtc_1>;
+		offset = <0x13>;
+	};
+
 	adc@0 {
 		compatible = "sandbox,adc";
 		vdd-supply = <&buck2>;