MAINTAINERS: Add drm/tinydrm maintainer entry

Add myself as drivers/gpu/drm/tinydrm maintainer.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502645790-14944-1-git-send-email-noralf@tronnes.org