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: Dinh Nguyen <[e-mail]>
Cc: Vince Bridgers <[e-mail]>
CC: David S. Miller <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

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

···
 						  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);