ARM: sunxi: Fix Mini X-plus device tree build

Commit b00adbe0 ("ARM: sunxi: Rename uart nodes to serial") changed the
node names in the DTSI, changes that were not accordingly made to the
Mini X-Plus device tree. This breakage slipped through because it was
not properly declared in the Makefile.

Fix both issues.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>