tty: serial: imx: allow breaks to be received when using dma

This allows me to login after sending a break when service
serial-getty@ttymxc0.service is running

The "tty_insert_flip_char(port, 0, TTY_BREAK)" in clear_rx_errors
fixes this by allowing the higher layers to see a break.

Also, call uart_handle_break to handle possible
"secure attention key."

FYI: Martin said the ROM sdma firmware works with this patch,
but external sdma firmware still does not send breaks on a
i.mx6UL

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Tested-by: Martin Hicks <mort@bork.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>