MAINTAINERS: Add Jernej Škrabec as a reviewer for Allwinner SoCs support

Jernej has helped a lot by reviewing patches recently, so let's make it
official.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://lore.kernel.org/r/20201009074423.10708-1-maxime@cerno.tech