MAINTAINERS: add Ralink pinctrl driver

Add a new section for the Ralink pinctrl driver and add me and Sergio as
the maintainers.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20220414173916.5552-9-arinc.unal@arinc9.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>