simplefb: Add simplefb MAINTAINERS entry

During the discussion about adding clock handling code to simplefb, it became
clear that simplefb currently does not have an active maintainer.

I've discussed this with Stephen Warren <swarren@wwwdotorg.org>, the original
author of simplefb, and with his permisson I'm picking up maintainership of
simplefb.

Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>