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/test/dm/Makefile b/test/dm/Makefile
index 6a36cc0..7b4dd6e 100644
--- a/test/dm/Makefile
+++ b/test/dm/Makefile
@@ -63,4 +63,5 @@
 obj-$(CONFIG_VIRTIO_SANDBOX) += virtio.o
 obj-$(CONFIG_DMA) += dma.o
 obj-$(CONFIG_DM_MDIO) += mdio.o
+obj-$(CONFIG_DM_MDIO_MUX) += mdio_mux.o
 endif