dm: core: Update ofnode to read binman-style flash entry

At present ofnode_read_fmap_entry() reads a flash map entry in a format
which is not supported by binman. To allow use to use binman-format
descriptions, update this function.

Also add a simple test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox64.dts b/arch/sandbox/dts/sandbox64.dts
index 0e32fda..2c6d351 100644
--- a/arch/sandbox/dts/sandbox64.dts
+++ b/arch/sandbox/dts/sandbox64.dts
@@ -17,7 +17,7 @@
 		stdout-path = "/serial";
 	};
 
-	cros_ec: cros-ec@0 {
+	cros_ec: cros-ec {
 		reg = <0 0 0 0>;
 		compatible = "google,cros-ec-sandbox";
 
@@ -25,23 +25,23 @@
 		 * This describes the flash memory within the EC. Note
 		 * that the STM32L flash erases to 0, not 0xff.
 		 */
-		#address-cells = <1>;
-		#size-cells = <1>;
-		flash@8000000 {
-			reg = <0x08000000 0x20000>;
+		flash {
+			image-pos = <0x08000000>;
+			size = <0x20000>;
 			erase-value = <0>;
-			#address-cells = <1>;
-			#size-cells = <1>;
 
 			/* Information for sandbox */
 			ro {
-				reg = <0 0xf000>;
+				image-pos = <0>;
+				size = <0xf000>;
 			};
 			wp-ro {
-				reg = <0xf000 0x1000>;
+				image-pos = <0xf000>;
+				size = <0x1000>;
 			};
 			rw {
-				reg = <0x10000 0x10000>;
+				image-pos = <0x10000>;
+				size = <0x10000>;
 			};
 		};
 	};