arm64: dts: meson-axg: s400: add cts-rts to the bluetooth uart

The uart used with bluetooth chipset on the s400 has flow control
available. Let's enable it.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>