tty/serial: meson_uart: update to stable bindings

This patch handle the stable UART bindings but also keeps compatibility
with the legacy non-stable bindings until all boards uses them.

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Helmut Klein <hgkr.klein@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>