serial: imx: add support for DTE mode

The uart works in the DCE mode by default, but sometime we need it
works at the DTE mode.

This patch adds the support for the DTE mode.

Signed-off-by: Huang Shijie <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

Documentation/devicetree/bindings/tty/serial/fsl-imx-uart.txt

···
 Optional properties:
 - fsl,uart-has-rtscts : Indicate the uart has rts and cts
 - fsl,irda-mode : Indicate the uart supports irda mode
+- fsl,dte-mode : Indicate the uart works in DTE mode. The uart works
+                  is DCE mode by default.
 
 Example:
 
···
 	reg = <0x73fbc000 0x4000>;
 	interrupts = <31>;
 	fsl,uart-has-rtscts;
+	fsl,dte-mode;
 };

drivers/tty/serial/imx.c

···
 	unsigned int		old_status;
 	int			txirq, rxirq, rtsirq;
 	unsigned int		have_rtscts:1;
+	unsigned int		dte_mode:1;
 	unsigned int		use_irda:1;
 	unsigned int		irda_inv_rx:1;
 	unsigned int		irda_inv_tx:1;
···
 
 	ufcr = readl(sport->port.membase + UFCR);
 	ufcr = (ufcr & (~UFCR_RFDIV)) | UFCR_RFDIV_REG(div);
+	if (sport->dte_mode)
+		ufcr |= UFCR_DCEDTE;
 	writel(ufcr, sport->port.membase + UFCR);
 
 	writel(num, sport->port.membase + UBIR);
···
 
 	if (of_get_property(np, "fsl,irda-mode", NULL))
 		sport->use_irda = 1;
+
+	if (of_get_property(np, "fsl,dte-mode", NULL))
+		sport->dte_mode = 1;
 
 	sport->devdata = of_id->data;