tty:serial:imx: Reduce RS485 turnarround time by requesting low latency from pm_qos

The long irq latencies measured on the RS485 ports could be tracked down to the time it takes the cpu to wake up from idle. With the pm_qos framework it is possible to request low latencies from the powermanagement, so it would select another idle state with shorter wakeup latency (on i.MX8MP cpu-pd-wait (slow) vs WFI (fast)). With this the interrupts are executed much faster and the measured latency on the RS485 port, between last stop bit and the release of TX_EN was reduced from up to 300us to 8-30us.

Closes PBB-39