arm64: dts: meson-gxbb-nanopi-k2: add missing model

This fixes the following DT schemas check errors:
meson-gxbb-nanopi-k2.dt.yaml: /: 'model' is a required property

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>