NET: DW: fix regression for ARC boards

The commit
642b80d2 ("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>