MAINTAINERS: input: add mattijs for mt6779-keypad

MAINTAINERS: input: add mattijs for mt6779-keypad

As stated in [1]:
Fengping has no longer interest and time to maintain this driver so he
agreed to transfer maintainership over to me.

Add a dedicated maintainer entry as well for the driver to make sure
that I can help with patch reviews.

[1] https://lore.kernel.org/r/20220421140255.2781505-1-mkorpershoek@baylibre.com

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20220720-mt8183-keypad-v2-1-6d42c357cb76@baylibre.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>