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

Document drive strength settings for Bitmain BM1880 SoC.

Signed-off-by: Manivannan Sadhasivam <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

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

···
                   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 {