drivers: wifi: nxp: add CONFIG_NXP_WIFI_BUILD_ONLY_MODE for CI testing

Add CONFIG_NXP_WIFI_BUILD_ONLY_MODE, which allows the NXP WiFi driver to
be built without a binary blob. The built application will not be
functional, but this enables continuous integration to run upstream.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>