MAINTAINERS: Add mmiotrace entry

The Nouveau maintainers would like to follow and review mmiotrace
changes as well, so create a separate entry for that code. The high
level bits are living in the tracing code, the low level bits in the
x86 code.

Acked-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Pekka Paalanen <ppaalanen@gmail.com>
Acked-by: karol herbst <karolherbst@gmail.com>
Cc: linux-kernel@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>