asm-generic/irqstat: Add optional __nmi_count member

Add an optional __nmi_count member to irq_cpustat_t so more architectures
can use the generic version.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.501611990@linutronix.de