dt-bindings: pinctrl: Document the i.MX50 IOMUXC binding

AFAICS from the i.MX50 Reference Manual, the i.MX50 IOMUXC works the
same as the one in i.MX51, so I copied fsl,imx51-pinctrl.txt and changed
the text to imx50.

Signed-off-by: Jonathan Neuschäfer <[e-mail]>
Cc: Dong Aisheng <[e-mail]>
Cc: Shawn Guo <[e-mail]>
Reviewed-by: Fabio Estevam <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/devicetree/bindings/pinctrl/fsl,imx50-pinctrl.txt

···
+* Freescale IMX50 IOMUX Controller
+
+Please refer to fsl,imx-pinctrl.txt in this directory for common binding part
+and usage.
+
+Required properties:
+- compatible: "fsl,imx50-iomuxc"
+- fsl,pins: two integers array, represents a group of pins mux and config
+  setting. The format is fsl,pins = <PIN_FUNC_ID CONFIG>, PIN_FUNC_ID is a
+  pin working on a specific function, CONFIG is the pad setting value like
+  pull-up for this pin. Please refer to imx50 datasheet for the valid pad
+  config settings.
+
+CONFIG bits definition:
+PAD_CTL_HVE			(1 << 13)
+PAD_CTL_HYS			(1 << 8)
+PAD_CTL_PKE			(1 << 7)
+PAD_CTL_PUE			(1 << 6)
+PAD_CTL_PUS_100K_DOWN		(0 << 4)
+PAD_CTL_PUS_47K_UP		(1 << 4)
+PAD_CTL_PUS_100K_UP		(2 << 4)
+PAD_CTL_PUS_22K_UP		(3 << 4)
+PAD_CTL_ODE			(1 << 3)
+PAD_CTL_DSE_LOW			(0 << 1)
+PAD_CTL_DSE_MED			(1 << 1)
+PAD_CTL_DSE_HIGH		(2 << 1)
+PAD_CTL_DSE_MAX			(3 << 1)
+PAD_CTL_SRE_FAST		(1 << 0)
+PAD_CTL_SRE_SLOW		(0 << 0)
+
+Refer to imx50-pinfunc.h in device tree source folder for all available
+imx50 PIN_FUNC_ID.