sunxi: Add boards/sunxi and arch/arm/mach-sunxi to sunxi MAINTAINERS entry

Recently some sunxi related code was moved to arch/arm/mach-sunxi, but
the MAINTAINERS entry was not updated to reflect this. Add this, and
the board level boards/sunxi directory to our entry.

While at it, also update its status, to reflect the current active
maintainership.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>