Use kernel/Kconfig.preempt for ARM

before this patch ARM had it's own definition of CONFIG_PREEMPT.
Instead of that use the general definition provided in
kernel/Kconfig.preempt.

This patch changes the available options in *config.  Namely PREEMPT
isn't a stand alone bool anymore, but part of a 'choice' the
following Kconfig symbols are added:

	PREEMPT_NONE
	PREEMPT_VOLUNTARY

Selecting PREEMPT_NONE now produces exactly the code as not selecting
PREEMPT before (apart from a comment in
/sys/kernel/debug/tracing/(per_cpu/cpuX/)?trace).  The only difference
PREEMPT_VOLUNTARY does is that might_resched might reschedule.  Both
should't introduce regressions.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Russell King <linux@arm.linux.org.uk>