cpm_uart: Modem control lines support

This patch replaces the get_mctrl/set_mctrl stubs with modem control line
read/write access through the GPIO lib.

Available modem control lines are described in the device tree using GPIO
bindings. The driver expect a GPIO pin for each of the CTS, RTS, DCD, DSR,
DTR and RI signals. Unused control lines can be left out.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

Documentation/powerpc/dts-bindings/fsl/cpm_qe/serial.txt

@@ -7,6 +7,15 @@ Currently defined compatibles:
 - fsl,cpm2-scc-uart
 - fsl,qe-uart
 
+Modem control lines connected to GPIO controllers are listed in the gpios
+property as described in booting-without-of.txt, section IX.1 in the following
+order:
+
+CTS, RTS, DCD, DSR, DTR, and RI.
+
+The gpios property is optional and can be left out when control lines are
+not used.
+
 Example:
 
 	serial@11a00 {
@@ -18,4 +27,6 @@ Example:
 		interrupt-parent = <&PIC>;
 		fsl,cpm-brg = <1>;
 		fsl,cpm-command = <00800000>;
+		gpios = <&gpio_c 15 0
+			 &gpio_d 29 0>;
 	};