gpio: mxc: Support module build

Change config to tristate, add module device table, module author,
description and license to support module build for i.MX GPIO driver.

As this is a SoC GPIO module, it provides common functions for most
of the peripheral devices, such as GPIO pins control, secondary
interrupt controller for GPIO pins IRQ etc., without GPIO driver, most
of the peripheral devices will NOT work properly, so GPIO module is
similar with clock, pinctrl driver that should be loaded ONCE and
never unloaded.

Since MXC GPIO driver needs to have init function to register syscore
ops once, here still use subsys_initcall(), NOT module_platform_driver().

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1600320829-1453-1-git-send-email-Anson.Huang@nxp.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>