serial: imx: use hrtimers for rs485 delays

serial: imx: use hrtimers for rs485 delays

This patch imitates 6e0a5de213 ("serial: 8250: Use hrtimers for
rs485 delays") in replacing the previously used classic timers
with hrtimers. The old way provided a too coarse resolution on
systems with configs of less than 1000 HZ.

Use of hrtimers addresses this and can be easily extended to
support microsecond resolution in future when support
for this arrives upstream.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20200714093012.21621-3-uwe@kleine-koenig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>