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: NAdrian Alonso <aalonso@freescale.com>
Acked-by: NShawn Guo <shawnguo@kernel.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>