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>