V4 Zynq/ZynqMP UART driver

[chrisj at rtems.org]

Hi,

This is version 4 of the UART driver changes. This change adds:

- Wait for the TX FITO to empty and the TX state to be idle
  before setting the baudrate in the first open and setting
  the attributes.

- Read any data in the RX FIFO.

The change commit log highlights the purpose of the change.

I have tested this on zynqmp with no UART issues and I have tested
it on a zynq application. All testing is in hardware.

Chris