soc: xtensa/nxp_adsp: put guard in Kconfig.defconfig

This adds a if CONFIG_SOC_FAMILY_NXP_ADSP guard in
Kconfig.defconfig for nxp_adsp. Or else all of its default
get applied everywhere. For example, qemu_xtensa fails
kernel.logging.message_capture tests because
CONFIG_TEST_LOGGING_DEFAULTS is disabled in
nxp_adsp/Kconfig.defconfig which should not have applied
to qemu_xtensa at all. So put a guard in there.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>