test/py: mii: Add tests for mii command

Add below test cases for mii commands:
mii_info -To display MII PHY info
mii_list - To list MII devices
mii_set_device - To set MII device
mii_read - To reads register from MII PHY address
mii_dump - To display data from MII PHY address

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