arm64: dts: meson-gxbb: nanopi-k2: Enable Bluetooth

The NanoPi K2 has a AP6212 WiFi+BT combo module. The WiFi portion is
already enabled. The BT part is connected via UART and I2S.

Enable the UART and add a device node describing the Bluetooth portion
of the module.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://lore.kernel.org/r/20210712135919.32059-1-wens@kernel.org