sunxi: add support for Banana Pi M2 Plus board

Banana Pi M2 Plus is an Allwinner H3-based SBC by Sinovoip, which has
already mainline device tree file that have landed into U-Boot source
tree.

Add a defconfig file for it and add the MAINTAINERS items.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>