arm/xilinx: Fix zynq-uart interrupt receive

- Trigger on a single character entering the RX FIFO

- Disable the RX timeout

- Send up to a FIFO full of data

bsps/include/dev/serial/zynq-uart.h

@@ -52,6 +52,7 @@ extern "C" {
 typedef struct {
   rtems_termios_device_context base;
   volatile struct zynq_uart *regs;
+  int tx_queued;
   bool transmitting;
   rtems_vector_number irq;
 } zynq_uart_context;