NET: DW: fix regression for ARC boards

The commit
642b80d256e ("net: designware: drop compatible altr, socfpga-stmmac")
breaks designware ethernet for all ARC boards. It removes
"altr, socfpga-stmmac" compatible from "drivers/net/designware.c"
without changing compatible in the boards which use it.

Fix that by adding "snps,arc-dwmac-3.70a" compatible string to
"drivers/net/designware.c" and using it in ARC boards device tree.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
diff --git a/arch/arc/dts/hsdk.dts b/arch/arc/dts/hsdk.dts
index 7028050..34ef3a6 100644
--- a/arch/arc/dts/hsdk.dts
+++ b/arch/arc/dts/hsdk.dts
@@ -71,7 +71,7 @@
 
 	ethernet@f0008000 {
 		#interrupt-cells = <1>;
-		compatible = "altr,socfpga-stmmac";
+		compatible = "snps,arc-dwmac-3.70a";
 		reg = <0xf0008000 0x2000>;
 		phy-mode = "gmii";
 	};