dt-bindings: imx: update pinctrl doc for imx6sll

Add pinctrl binding doc update for imx6sll.

Signed-off-by: Bai Ping <ping.bai@nxp.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Acked-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>