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

Document slew-rate DT binding for SAMA7G5.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1611747945-29960-2-git-send-email-claudiu.beznea@microchip.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -35,9 +35,11 @@ ioset settings. Use the macros from boot/dts/<soc>-pinfunc.h file to get the
 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.
+- 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.
+	- for microchip,sama7g5-pinctrl only:
+		- 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.