ARCv2: intc: default all interrupts to priority 1

ARCv2: intc: default all interrupts to priority 1

ARC HS Cores support configurable multiple interrupt priorities of upto
16 levels. In commit e007b1b43c5c1 ("ARCv2: intc: Allow interruption by
lowest priority interrupt") we switched to 15 which seems a bit
excessive given that there would be rare hardware implementing so many
preemption levels AND running Linux. It would seem that 2 levels will be
more common so switch to 1 as the default priority level. This will be
the "lower" priority level saving 0 for implementing NMI style support.

This scheme also works in systems with more than 2 prioity levels as
well.

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