gpio: mxc: Convert the driver to DT-only

Since 5.10-rc1 i.MX is a devicetree-only platform, so simplify the code
by removing the unused non-DT support.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20201117105917.27591-1-festevam@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>