dt-bindings: add documentation for slew rate

Add documentation for slew rate.
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -100,6 +100,7 @@ DRIVE_STRENGTH (3 << 5): indicate the drive strength of the pin using the
 				11 - High
 OUTPUT		(1 << 7): indicate this pin need to be configured as an output.
 OUTPUT_VAL	(1 << 8): output val (1 = high, 0 = low)
+SLEWRATE	(1 << 9): slew rate of the pin: 0 = disable, 1 = enable
 DEBOUNCE	(1 << 16): indicate this pin needs debounce.
 DEBOUNCE_VAL	(0x3fff << 17): debounce value.