clocksource: Add a Kconfig menu

Move the clocksource Kconfig entries into their own menu, so that they
don't pollute the main device driver menu.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: http://lkml.kernel.org/r/20140616114845.343e9960@endymion.delvare


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>