ARC: [intc] don't mask all IRQ by default

Hardware keeps them enabled on reset, and Linux needs to keep status quo.
Any spurious interrupts will be reported/blocked by genirq.

This helps remove a SMP IRQ quirk (next commit), where a peripheral IRQ
is hard wired to core0, and request_irq()->unmask() happens on core1,
keeping the IRQ masked on core0, needing an explicit unmask.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>