net: stmmac: Introduce selftests support

We add support for selftests on stmmac driver with 9 basic sanity checks
for now:
	- MAC Loopback
	- PHY Loopback
	- MMC Counters
	- EEE
	- Hash Filter Multicast
	- Perfect Filter Unicast
	- Multicast Filter All
	- Unicast Filter All
	- Flow Control

This allows for fast tracking of regressions in the driver and helps in
spotting mis-configuration of HW.

Changes from v1:
	- Fix build error as module (David)
	- Check for link status before running tests
Changes from RFC v2:
	- Return proper error code in stmmac_test_mmc (Corentin)
	- Use only 1 MMC counter in stmmac_test_mmc (Alexandre)
Changes from RFC v1:
	- Change test_loopback to test_mac_loopback (Andrew)
	- Change timeout to retries (Andrew)
	- Add MC/UC filter tests (Andrew)
	- Only test in offline mode (Andrew)
	- Do not call phy_loopback twice (Alexandre)

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Corentin Labbe <clabbe.montjoie@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>