dt-bindings: pinctrl: Document pinconf bindings for BM1880 SoC

Document pinconf bindings for Bitmain BM1880 SoC.
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/bitmain,bm1880-pinctrl.txt

@@ -14,7 +14,8 @@ phrase "pin configuration node".
 The pin configuration nodes act as a container for an arbitrary number of
 subnodes. Each of these subnodes represents some desired configuration for a
 pin, a group, or a list of pins or groups. This configuration for BM1880 SoC
-includes only pinmux as there is no pinconf support available in SoC.
+includes pinmux and various pin configuration parameters, such as pull-up,
+slew rate etc...
 
 Each configuration node can consist of multiple nodes describing the pinmux
 options. The name of each subnode is not important; all subnodes should be
@@ -84,6 +85,22 @@ Required Properties:
                   gpio66, gpio67, eth1, i2s0, i2s0_mclkin, i2s1, i2s1_mclkin,
                   spi0
 
+Optional Properties:
+
+- bias-disable:  No arguments. Disable pin bias.
+- bias-pull-down: No arguments. The specified pins should be configured as
+                  pull down.
+- bias-pull-up:   No arguments. The specified pins should be configured as
+                  pull up.
+- input-schmitt-enable: No arguments: Enable schmitt trigger for the specified
+                  pins
+- input-schmitt-disable: No arguments: Disable schmitt trigger for the specified
+                  pins
+- slew-rate:      Integer. Sets slew rate for the specified pins.
+                  Valid values are:
+                  <0>  - Slow
+                  <1>  - Fast
+
 Example:
         pinctrl: pinctrl@400 {
                 compatible = "bitmain,bm1880-pinctrl";