dt-bindings: add Marvell Dove LCD controller documentation

Add the Marvell Dove LCD controller DT binding documentation.  The
clock names used here are intentionally taken from the specification
for the Dove SoC.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Documentation/devicetree/bindings/drm/armada/marvell,dove-lcd.txt

+Device Tree bindings for Armada DRM CRTC driver
+
+Required properties:
+ - compatible: value should be "marvell,dove-lcd".
+ - reg: base address and size of the LCD controller
+ - interrupts: single interrupt number for the LCD controller
+ - port: video output port with endpoints, as described by graph.txt
+
+Optional properties:
+
+ - clocks: as described by clock-bindings.txt
+ - clock-names: as described by clock-bindings.txt
+	"axiclk" - axi bus clock for pixel clock
+	"plldivider" - pll divider clock for pixel clock
+	"ext_ref_clk0" - external clock 0 for pixel clock
+	"ext_ref_clk1" - external clock 1 for pixel clock
+
+Note: all clocks are optional but at least one must be specified.
+Further clocks may be added in the future according to requirements of
+different SoCs.
+
+Example:
+
+	lcd0: lcd-controller@820000 {
+		compatible = "marvell,dove-lcd";
+		reg = <0x820000 0x1000>;
+		interrupts = <47>;
+		clocks = <&si5351 0>;
+		clock-names = "ext_ref_clk_1";
+	};