ARCv2: intc: Set default priority for all core interrupts

After reset all interrupts in the core interrupt controller has
the highest priority P0. If the platform supports Fast IRQs and
has more than 1 banks of registers then CPU automatically switch
banks of registers when P0 interrupt comes.

The problem is that the kernel expects that by default switching
of banks is not used by all interrupts. It is necessary to set a
default nonzero priority for all available interrupts to avoid
undefined behaviour.

Signed-off-by: Yuriy Kolerov <yuriy.kolerov@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>