serial: imx: Also enable the aging timer in PIO mode

This allows to increase the RX waterlevel which allows to delay the RRDY
irq. The desired effect is that less irqs are needed to handle
characters and so reduce irq count of the system.

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