Enable building the PL061 GPIO driver as a module.

This does change the initcall level when built-in. This shouldn't be a
problem as any user should support deferred probe by now. A scan of DT
based platforms at least didn't reveal any users that would be a
problem.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>