dt-bindings: pinctrl: Document drive strength settings for BM1880 SoC

Document drive strength settings 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

@@ -100,6 +100,17 @@ Optional Properties:
                   Valid values are:
                   <0>  - Slow
                   <1>  - Fast
+- drive-strength: Integer. Selects the drive strength for the specified
+                  pins in mA.
+                  Valid values are:
+                  <4>
+                  <8>
+                  <12>
+                  <16>
+                  <20>
+                  <24>
+                  <28>
+                  <32>
 
 Example:
         pinctrl: pinctrl@400 {