MAINTAINERS: Add an entry for PREEMPT_RT.

Add a maintainers entry now that the PREEMPT_RT bits are merged. Steven
volunteered and asked for the list.

There are no files associated with this entry since it is spread over the
kernel. It serves as entry for people knowing what they look for.  There is
a keyword added so if PREEMPT_RT is mentioned somewhere, then the entry
will be picked up.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Pavel Machek <pavel@denx.de>
Link: https://lore.kernel.org/all/20241015151132.Erx81G9f@linutronix.de