dt-bindings: pinctrl: at91-pio4: add slew-rate

Document slew-rate DT binding for SAMA7G5.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
diff --git a/doc/device-tree-bindings/pinctrl/atmel,at91-pio4-pinctrl.txt b/doc/device-tree-bindings/pinctrl/atmel,at91-pio4-pinctrl.txt
index 9252dc1..6e936a0 100644
--- a/doc/device-tree-bindings/pinctrl/atmel,at91-pio4-pinctrl.txt
+++ b/doc/device-tree-bindings/pinctrl/atmel,at91-pio4-pinctrl.txt
@@ -25,9 +25,10 @@
 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.
+- GENERIC_PINCONFIG: generic pinconfig options to use:
+	- bias-disable, bias-pull-down, bias-pull-up, drive-open-drain,
+	  input-schmitt-enable, input-debounce
+	- slew-rate: 0 - disabled, 1 - enabled (default)
 - atmel,drive-strength: 0 or 1 for low drive, 2 for medium drive and 3 for
 high drive. The default value is low drive.