gpio: of: Build fails if CONFIG_OF_DYNAMIC enabled without CONFIG_OF_GPIO

gpio: of: Build fails if CONFIG_OF_DYNAMIC enabled without CONFIG_OF_GPIO

The symbol 'gpio_of_notifier' doesn't exist without both CONFIG_OF_GPIO
and CONFIG_OF_DYNAMIC enabled, but is referenced when only
CONFIG_OF_DYNAMIC is enabled.

This broke building with 'make ARCH=um allyesconfig':
---------------
/usr/bin/ld: drivers/gpio/gpiolib.o: in function `gpiolib_dev_init':
./drivers/gpio/gpiolib.c:5293: undefined reference to `gpio_of_notifier'
collect2: error: ld returned 1 exit status
---------------

Fixes: 63636d956c45 ("gpio: of: Add DT overlay support for GPIO hogs")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200425044655.166257-1-davidgow@google.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>