This is a non-critical patch, that makes configuring an rt kernel a
little nicer.
Comments are welcome of course.

Signed-off-by: John Kacur <jkacur at gmail dot com>

This patch makes the following changes.
1. Hides the visibility of PREEMPT_SOFTIRQS and PREEMPT_HARDIRQS if PREEMPT_RT
(Complete Preemption) is choosen, since these values are then set (selected) to
be "y".

2. Changes the PREEMPT_HARDIRQS to depend on PREEMPT_SOFTIRQS instead of
forcing PREEMPT_SOFTIRQS to y if PREEMPT_HARDIRQS is choosen. (this is somewhat
an abuse of select)

3. Minor spelling fix. (tat to that)