x86/tsc: Mark cyc2ns_init() and detect_art() __init

These two functions are only called by tsc_init(), which is an __init
function during boot time, so mark them __init as well.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1510135792-17429-1-git-send-email-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>