test: panel: Add a test for the panel uclass

At present this uclass has no tests. Add a simple one which checks the PWM
configuration, regulator and GPIO.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox_pmic.dtsi b/arch/sandbox/dts/sandbox_pmic.dtsi
index 403656f..5ecafaa 100644
--- a/arch/sandbox/dts/sandbox_pmic.dtsi
+++ b/arch/sandbox/dts/sandbox_pmic.dtsi
@@ -60,7 +60,7 @@
 		regulator-max-microvolt = <3300000>;
 	};
 
-	ldo1 {
+	ldo_1: ldo1 {
 		regulator-name = "VDD_EMMC_1.8V";
 		regulator-min-microvolt = <1800000>;
 		regulator-max-microvolt = <1800000>;