ARM: at91/at91-pinctrl documentation: add missing sam9x5 compatible string

Add missing "atmel,at91sam9x5-pinctrl" compatible string to the
documentation.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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

@@ -18,7 +18,7 @@ mode) this pin can work on and the 'config' configures various pad settings
 such as pull-up, multi drive, etc.
 
 Required properties for iomux controller:
-- compatible: "atmel,at91rm9200-pinctrl"
+- compatible: "atmel,at91rm9200-pinctrl" or "atmel,at91sam9x5-pinctrl"
 - atmel,mux-mask: array of mask (periph per bank) to describe if a pin can be
   configured in this periph mode. All the periph and bank need to be describe.