Various IMX platforms may have different imx_pmx_ops.gpio_set_direction
implementations, so let's make it platform specific callbacks instead of
the fixed common one.

Currently only VF610 platform implements it. No function level changes.

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>