MAINTAINERS: Add Jernej Škrabec as a reviewer for DE2

The newer Allwinner SoCs have a different layers controller than the older
ones. Jernej wrote that support and has been reviewing patches for a while
now, so let's make him a formal reviewer.

Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190919173020.11655-2-mripard@kernel.org