serial: zynq: Add support for slow emulation platform

On slow platforms not all baudrate setting is valid.
Check it directly in the driver and setup maximum possible
frequency.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/serial/serial_zynq.c

@@ -48,10 +48,16 @@ static void uart_zynq_serial_setbrg(const int port)
 	/* Calculation results. */
 	unsigned int calc_bauderror, bdiv, bgen;
 	unsigned long calc_baud = 0;
-	unsigned long baud = gd->baudrate;
+	unsigned long baud;
 	unsigned long clock = get_uart_clk(port);
 	struct uart_zynq *regs = uart_zynq_ports[port];
 
+	/* Covering case where input clock is so slow */
+	if (clock < 1000000 && gd->baudrate > 4800)
+		gd->baudrate = 4800;
+
+	baud = gd->baudrate;
+
 	/*                master clock
 	 * Baud rate = ------------------
 	 *              bgen * (bdiv + 1)