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

GMAC controller on HSDK boards supports 256 Hash Table size so we need to
add the multicast filter bins property. This allows for the Hash filter
to work properly using stmmac driver.

Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Acked-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>