MAINTAINERS: Add allwinner/sunxi driver directories

These drivers are sunxi platform-specific, and so are of interest to the
sunxi maintainers.

In fact, as there is no PHY driver maintainer, drivers/phy/allwinner had
no maintainer at all.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>