test: regmap: test Linux-compatible syscon_node_to_regmap()

Like Linux, syscon_node_to_regmap() allows a node to work as a syscon
provider without binding it to a syscon driver.  Test this.

Requested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 06d0e8c..3c25cb7 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -393,6 +393,14 @@
 			0x38 8>;
 	};
 
+	syscon@2 {
+		compatible = "simple-mfd", "syscon";
+		reg = <0x40 5
+			0x48 6
+			0x50 7
+			0x58 8>;
+	};
+
 	timer {
 		compatible = "sandbox,timer";
 		clock-frequency = <1000000>;