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/axs10x_mb.dtsi b/arch/arc/dts/axs10x_mb.dtsi
index 6d97de9..5b77642 100644
--- a/arch/arc/dts/axs10x_mb.dtsi
+++ b/arch/arc/dts/axs10x_mb.dtsi
@@ -53,7 +53,7 @@
 		};
 
 		ethernet@18000 {
-			compatible = "altr,socfpga-stmmac";
+			compatible = "snps,arc-dwmac-3.70a";
 			reg = < 0x18000 0x2000 >;
 			phy-mode = "gmii";
 			snps,pbl = < 32 >;
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";
 	};
diff --git a/drivers/net/designware.c b/drivers/net/designware.c
index 145eeac..0031370 100644
--- a/drivers/net/designware.c
+++ b/drivers/net/designware.c
@@ -852,6 +852,7 @@
 	{ .compatible = "amlogic,meson-gxbb-dwmac" },
 	{ .compatible = "amlogic,meson-axg-dwmac" },
 	{ .compatible = "st,stm32-dwmac" },
+	{ .compatible = "snps,arc-dwmac-3.70a" },
 	{ }
 };