serial: imx: Switch to nbcon console

serial: imx: Switch to nbcon console

Implements the necessary callbacks to switch the imx console driver to
perform as an nbcon console.

Add implementations for the nbcon consoles (write_atomic, write_thread,
driver_enter, driver_exit) and add CON_NBCON to the initial flags.

The legacy code is kept in order to easily switch back to legacy mode
by defining CONFIG_SERIAL_IMX_LEGACY_CONSOLE.

Signed-off-by: Esben Haabendal <esben@geanix.com>
Link: https://lore.kernel.org/r/20240913-serial-imx-nbcon-v3-1-4c627302335b@geanix.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>