pinctrl: imx: switch to use the generic pinmux property

The generic pinmux property seems to be more suitable for IMX.
So we change to use 'pinmux' instead of 'pins'.

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