serial: imx: document functions that are called with port.lock taken

Consistently indicate being called with irqs off and the port lock taken
for all functions that this applies to.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>