test/py: mdio: Add tests for mdio command

Add below test cases for mdio commands:
mdio_list - To list MDIO buses
mdio_read - To read PHY's register at <devad>.<reg>
mdio_write - To write PHY's register at <devad>.<reg>

Signed-off-by: Love Kumar <love.kumar@amd.com>
Reviewed-by: Tom Rini <trini@konsulko.com>