pinctrl: freescale: imx: allow mux_reg offset zero

Allow mux_reg offset zero to be a valid pin_id, on imx7d
mux_conf reg offset is zero for iomuxc-lspr controller

Signed-off-by: Adrian Alonso <aalonso@freescale.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>