powerpc/40x: Add missing select OF_GPIO_MM_GPIOCHIP

Since the split of the legacy of_mm_gpio_chip to a separate
file and a specific build configuration option, the users
must select it when needed. The PowerPC 40x code misses this,
so we have to add the select here.

Fixes: a99cc668

 ("gpiolib: split of_mm_gpio_chip out of linux/of_gpio.h")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>