serial/ns16550: Precision clock synthesizer

Set the FIFO control register while DLAB == 1 in the line control
register.  At least on the QorIQ T4240 the driver still works with the
re-ordered FIFO control register access.

bsps/include/libchip/ns16550.h

@@ -69,6 +69,7 @@ typedef struct {
   uint32_t clock;
   uint32_t initial_baud;
   bool has_fractional_divider_register;
+  bool has_precision_clock_synthesizer;
   uint8_t modem_control;
   uint8_t line_control;
   uint32_t baud_divisor;