pinctrl: dt-binding: at91-pio4: add drive-strength property

Add the drive-strength property whose valid values are
ATMEL_PIO_DRVSTR_LO, ATMEL_PIO_DRVSTR_ME, ATMEL_PIO_DRVSTR_HI.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>

Author: ldesroches

Documentation/devicetree/bindings/pinctrl/atmel,at91-pio4-pinctrl.txt

@@ -35,7 +35,12 @@ right representation of the pin.
 Optional properties:
 - GENERIC_PINCONFIG: generic pinconfig options to use, bias-disable,
 bias-pull-down, bias-pull-up, drive-open-drain, input-schmitt-enable,
-input-debounce, output-low, output-high.
+input-debounce, output-low, output-high, drive-strength.
+
+Concerning the drive-strength property, the values accepted are
+ATMEL_PIO_DRVSTR_LO, ATMEL_PIO_DRVSTR_ME, ATMEL_PIO_DRVSTR_HI. The current
+depends on VDDIOP. You can find the details in the DC Characteristics table
+from the datasheet.
 
 Example: