net: stmmac: make reset control an optional requirement

Not having a reset control line to the ethernet controller should not be a
hard failure. Instead, add support for deferred probing and just print out
a debug statement.
Signed-off-by: NDinh Nguyen <dinguyen@opensource.altera.com>
Cc: Vince Bridgers <vbridger@opensource.altera.com>
CC: David S. Miller <davem@davemloft.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c

@@ -91,7 +91,9 @@ static int socfpga_dwmac_parse_data(struct socfpga_dwmac *dwmac, struct device *
 						  STMMAC_RESOURCE_NAME);
 	if (IS_ERR(dwmac->stmmac_rst)) {
 		dev_info(dev, "Could not get reset control!\n");
-		return -EINVAL;
+		if (PTR_ERR(dwmac->stmmac_rst) == -EPROBE_DEFER)
+			return -EPROBE_DEFER;
+		dwmac->stmmac_rst = NULL;
 	}
 
 	dwmac->interface = of_get_phy_mode(np);