x86/numachip: Add const and __initconst to numachip2_clockevent

Make this const as it is only used during a copy operation and add
__initconst as this usage is during the initialization phase.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: julia.lawall@lip6.fr
Cc: daniel.lezcano@linaro.org
Link: http://lkml.kernel.org/r/1504030631-10812-1-git-send-email-bhumirks@gmail.com