MAINTAINERS: Make Dennis the percpu tree maintainer

Dennis rewrote a significant portion of the percpu allocator and has
shown that he can respond in a timely and helpful manner when issues
are reported against percpu allocator.

Let's make Dennis the percpu tree maintainer.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Christoph Lameter <cl@linux.com>