software/kernel/liteuart.c: added a workqueue to avoid blocking timer callback

[trabucayre]

With a continuous UART RX stream at high speed, timer callback took a too long time before finishing, or don't finish.
As the result some interrupts are blocked and the kernel hang with the need to force stop/start the computer.

This PR adds a workqueue to delegate RX FIFO read out of the timer callback.

To avoid to adds to long latency, this workqueue is only used after a 256 read sequence. This keep the default behavior in a normal uses.

Note to myself/TODO/FIXME: When the read is transfered to the workqueue, timer callback must avoid to call mod_timer and let the workqueue to do this task at the end of execution.

[enjoy-digital]

Thanks @trabucayre, this is merged.