MAINTAINERS: Add X86 MM entry

Dave, Andy and Peter are de facto overseing the mm parts of X86. Add an
explicit maintainers entry.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Ingo Molnar <mingo@kernel.org>