MAINTAINERS: Add entry for VKMS

Add maintainers and reviewers for VKMS driver

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190113204038.bvdc6d5tyxjz6bzf@smtp.gmail.com