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/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" },
 	{ }
 };