clocksource: Fix permissions for available_clocksource

File permissions for
/sys/devices/system/clocksource/clocksource0/available_clocksource
are 600 which allows write access. But this is in fact a read only
file. So change permissions to 400.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>