dt-bindings: pinctrl: at91-pio4: add property for drive strength

Add drive strength property which is equivalent with the one in Linux
Signed-off-by: NEugen Hristev <eugen.hristev@microchip.com>

doc/device-tree-bindings/pinctrl/atmel,at91-pio4-pinctrl.txt

@@ -28,6 +28,8 @@ 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.
+- atmel,drive-strength: 0 or 1 for low drive, 2 for medium drive and 3 for
+high drive. The default value is low drive.
 
 Example: