zynq-uart: Fix set_attributes implementation

The zynq-uart set_attributes implementation was configured to always
return false which causes spconsole01 to fail. This restores the
disabled implementation which sets the baud rate registers
appropriately and allows spconsole01 to pass. This also expands the
set_attributes functionality to allow setting of the stop bits,
character width, and parity.

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

@@ -78,6 +78,13 @@ void zynq_uart_write_polled(
   */
 void zynq_uart_reset_tx_flush(zynq_uart_context *ctx);
 
+int zynq_cal_baud_rate(
+  uint32_t  baudrate,
+  uint32_t* brgr,
+  uint32_t* bauddiv,
+  uint32_t  modereg
+);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */