tty:serial:imx: fix uart latency for rs485 on mx6ull

There is too big latency (up to 3ms) in rs485 mode on mx6ull during turning
To receive between disabling transmitter and enabling receiver.
It happens because of 3 sequential iterations of calling stop_tx (2 from irq
thread and 1 from hrtimer).
Irq thread has great latency on the slow mx6ull and RS485 is sensitive to
timings.
The patch involves hrtimer for calling stop_tx during waiting FIFO empty
in addition to existing calls from softirq to speed up the process.

To make new code apply only for specific devices and not to affect other
boards, a "rs485-has-rts-latency" property for uart device tree node was
added.

Signed-off-by: Mikhail Vanyulin <mikhail.vanyulin@rtsoft.de>