Add a missing property to GMAC node so that multicast filtering works
correctly.

Fixes: 556cc1c5 ("ARC: [axs101] Add support for AXS101 SDP (software development platform)")
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>