tests: drivers: dma: add mimxrt685 to scatter_gather platform_allow

The mimxrt685 has a different IP/driver from other NXP platforms
in the platform_allow list.

Signed-off-by: David Leach <[email protected]>

Author: dleach02

tests/drivers/dma/scatter_gather/testcase.yaml

@@ -8,6 +8,7 @@ tests:
       - intel_adsp/cavs25
       - frdm_k64f
       - mimxrt1010_evk
+      - mimxrt685_evk/mimxrt685s/cm33
       - mimxrt1060_evk/mimxrt1062/qspi
       - lpcxpresso55s36
       - native_sim