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/arch/Kconfig b/arch/Kconfig
index a946af8..949eb28 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -125,6 +125,7 @@
 	imply PCH
 	imply PHYLIB
 	imply DM_MDIO
+	imply DM_MDIO_MUX
 
 config SH
 	bool "SuperH architecture"