pinctrl: at91: Remove duplicate const

const declared twice. Fixes the following sparse warning:
drivers/pinctrl/pinctrl-at91.c:815:21: warning: duplicate const
drivers/pinctrl/pinctrl-at91.c:849:21: warning: duplicate const

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>