ARM: dts: sun8i: h3: Enable AP6212 BT on uart3 on Nanopi M1 Plus

The BT side of the AP6212 WiFi/BT combo module is connected to
uart3.

Enable BT on this board by enabling uart3 with using additionally
the cts and rts pins.

Signed-off-by: Philipp Rossak <embed3d@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>