gpio: omap: do not register driver in probe()

In the Linux kernel, the following vulnerability has been resolved: gpio: omap: do not register driver in probe() Commit 11a78b794496 ("ARM: OMAP: MPUIO wake updates") registers the omap_mpuio_driver from omap_mpuio_init(), which is called from omap_gpio_probe(). However, it neither makes sense to register drivers from probe() callbacks of other drivers, nor does the driver core allow registering drivers with a device lock already being held. The latter was revealed by commit dc23806a7c47 ("driver core: enforce device_lock for driver_match_device()") leading to a potential deadlock condition described in [1]. Additionally, the omap_mpuio_driver is never unregistered from the driver core, even if the module is unloaded. Hence, register the omap_mpuio_driver from the module initcall and unregister it in module_exit().