soc: nrf: fix NRF_DT_CHECK_GPIO_CTLR_IS_SOC

This was merged by mistake without being tested and is not working
properly. We need to avoid doing a BUILD_ASSERT() when the relevant
property is missing, because we can't use DT_GPIO_CTLR() on an
undefined property. Handle this with COND_CODE_1().

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>