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>