gpio: mxs: GPIO_MXS should not default to y unconditionally

Merely enabling CONFIG_COMPILE_TEST should not enable additional code.
To fix this, restrict the automatic enabling of GPIO_MXS to ARCH_MXS,
and ask the user in case of compile-testing.

Fixes: 6876ca31 ("gpio: mxs: add COMPILE_TEST support for GPIO_MXS")
Cc: <stable@vger.kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>