Current ARC SDP boards cannot reliably handle 1Gbit
Ethernet connections due to limitations in hardware.

To make sure networking is stable on the board we're
limiting phy to 100 Mbit.
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>