mailmap: Update email address for Quentin Perret

My @arm.com email address will stop working in a few weeks, so add an
entry to .mailmap so others have a way to reach me if they want to.

While at it, move some entries around to keep the file in alphabetical
order.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>