genirq: Provide an accessor for IRQ_PER_CPU flag

Adds an accessor function, irq_is_per_cpu(), for the flag IRQ_PER_CPU.

This is useful if you have an IRQ that is per-cpu in some hardware
implementations, and not in others (example: ARM PMU Interrupts), and
you want to handle both cases.

Change-Id: Ic176ee5b8f9a830c1db35cb939ec659a4cf3f938
Signed-off-by: Chris Smith <chris.smith@st.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>