of: unittest: make gpio overlay test dependent on CONFIG_OF_GPIO

Randconfig testing found compile errors in drivers/of/unittest.c if
CONFIG_GPIOLIB is not set because CONFIG_OF_GPIO depends on
CONFIG_GPIOLIB.  Make the gpio overlay test depend on CONFIG_OF_GPIO.

No code is modified, it is only moved to a different location and
protected with #ifdef CONFIG_OF_GPIO.  An empty
of_unittest_overlay_gpio() is added in the #else.

Fixes: f4056e7 ("of: unittest: add overlay gpio test to catch gpio hog problem")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Rob Herring <robh@kernel.org>