ARC: [SMP] unify cpu private IRQ requests (TIMER/IPI)

The current cpu-private IRQ registration is ugly as it requires need to
expose arch_unmask_irq() outside of intc code.
So switch to percpu IRQ APIs:
  -request_percpu_irq [boot core]
  -enable_percpu_irq  [all cores]

Encapsulated in helper arc_request_percpu_irq()

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

diff --cc arch/arc/include/asm/irq.h
Simple merge

diff --cc arch/arc/kernel/irq.c
Simple merge

diff --cc arch/arc/kernel/smp.c
Simple merge

diff --cc arch/arc/kernel/time.c
Simple merge