MAINTAINERS: Add entry for Himax HX8394 panel controller driver

Add myself as maintainer for the driver and devicetree bindings schema.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102230733.3506624-4-javierm@redhat.com