pinctrl: imx5: start numbering pad from 0

Unlike imx6q pinctrl driver that starts nubmering pad from 0, imx5
pinctrl drivers number pad from 1.  It causes problem/confusion when
driver accesses imx51_pinctrl_pads array using pin ID as the index.

Change imx51_pads and imx53_pads numbering start from 0.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>