test: dm: add a test for MDIO MUX DM uclass

Adds a test using a makeshift MDIO MUX.  The test is based on the existing
MDIO test.  It uses the last emulated PHY register to verify MUX selection.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 0dc26ac..403df5e 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -46,6 +46,16 @@
 
 	  This driver is used in for testing in test/dm/mdio.c
 
+config MDIO_MUX_SANDBOX
+	depends on DM_MDIO_MUX && MDIO_SANDBOX
+	default y
+	bool "Sandbox: Mocked MDIO-MUX driver"
+	help
+	  This driver implements dummy select/deselect ops mimicking a MUX on
+	  the MDIO bux.  It uses mdio_sandbox driver as parent MDIO.
+
+	  This driver is used for testing in test/dm/mdio.c
+
 menuconfig NETDEVICES
 	bool "Network device support"
 	depends on NET