MAINTAINERS: .mailmap: update Mattijs Korpershoek's email address

Update Mattijs Korpershoek's email address to @kernel.org.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Link: https://lore.kernel.org/r/20250428-keypad-maintainers-v1-1-4e9c4afba415@kernel.org


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>