ARC: [plat-axs10x]: Add missing multicast filter number to GMAC node

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

Fixes: 556cc1c5f5 ("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>

arch/arc/boot/dts/axs10x_mb.dtsi

@@ -78,6 +78,7 @@
 			interrupt-names = "macirq";
 			phy-mode = "rgmii";
 			snps,pbl = < 32 >;
+			snps,multicast-filter-bins = <256>;
 			clocks = <&apbclk>;
 			clock-names = "stmmaceth";
 			max-speed = <100>;