gpiolib: split of_mm_gpio_chip out of linux/of_gpio.h

This is a rarely used feature that has nothing to do with the
client-side of_gpio.h.

Split it out with a separate header file and Kconfig option
so it can be removed on its own timeline aside from removing
the of_gpio consumer interfaces.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>