ARC: [plat-hsdk]: Remove misplaced interrupt-cells property

"gmac" node stands for just an ordinary Ethernet controller,
which is by no means a provider of interrupts, i.e. it doesn't serve
as an interrupt controller, thus "#interrupt-cells" property doesn't
belong to it and so we remove it.

Fixes:
------------>8------------
  DTC     arch/arc/boot/dts/hsdk.dtb
arch/arc/boot/dts/hsdk.dts:207.23-235.5: Warning (interrupt_provider): /soc/ethernet@8000: '#interrupt-cells' found, but node is not an interrupt provider
arch/arc/boot/dts/hsdk.dtb: Warning (interrupt_map): Failed prerequisite 'interrupt_provider'
------------>8------------
Reported-by: Vineet Gupta <vgupta@kernel.org>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/boot/dts/hsdk.dts

@@ -205,7 +205,6 @@ dmac_cfg_clk: dmac-gpu-cfg-clk {
 		};
 
 		gmac: ethernet@8000 {
-			#interrupt-cells = <1>;
 			compatible = "snps,dwmac";
 			reg = <0x8000 0x2000>;
 			interrupts = <10>;