Add the binding for RX/TX fifo size of GMAC node.

Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Vineet Gupta <vgupta@synopsys.com>
Tested-by: 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>