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>