Reduce the maximum number of pins for the Rockchip RK3188, this driver
isn't supposed to manage all the GPIO pins in the system from a single
instance, instead it will attach to each one of the four available GPIO
controllers and only deal with one bank at time (32 pins per bank).

Rework part of the driver to take advantage of that, this simplify the
code a lot.

Also fix a bug in rk30_gpio_get_function() which wouldn't return the
correct values.

While here fix a typo in register name.