test: dm: add MDIO test

A very simple test for DM_MDIO, mimicks a register write/read through the
sandbox bus to a dummy PHY.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 6fba5a8..635f8d7 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -24,6 +24,16 @@
 	  This is currently implemented in net/mdio-uclass.c
 	  Look in include/miiphy.h for details.
 
+config MDIO_SANDBOX
+	depends on DM_MDIO && SANDBOX
+	default y
+	bool "Sandbox: Mocked MDIO driver"
+	help
+	  This driver implements dummy read/write/reset MDIO functions mimicking
+	  a bus with a single PHY.
+
+	  This driver is used in for testing in test/dm/mdio.c
+
 menuconfig NETDEVICES
 	bool "Network device support"
 	depends on NET