gpio: xgene-sb: Allow driver to be built with COMPILE_TEST

Allow driver to be built with COMPILE_TEST for better test coverage.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200512182721.55127-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>