dt-bindings: vendor-prefixes: Add mantix vendor prefix

Add prefix for Mantix Display Technology Co.,Ltd.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b6bcfe27d9393a07a5cdd82fcf211620c774a273.1597652012.git.agx@sigxcpu.org