MAINTAINERS: Add FUTEX SUBSYSTEM

Add a MAINTAINERS block for the FUTEX SUBSYSTEM which includes the core
kernel code, include headers, testing code, and Documentation. Excludes
arch files, and higher level test code.

I added tglx and mingo as M as they have made the tip commits and peterz
and myself as R.

Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Link: http://lkml.kernel.org/r/20170414224608.GA5180@fury


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